Enumerate reasons after because [closed]

This is more a question of style than grammar, and different style manuals may give different recommendations. I will use the recommendations of the Chicago Manual of Style (CMOS).

To begin with, CMOS would recommend that you either make the list items more integrated, or else make the list vertical.

If you want to retain the run-in format (which actually makes sense, since your list is quite simple), then you should at least (1) remove the colon and (2) rephrase the list items so that they together with the introductory material form a sentence. (The colon is only appropriate if the introductory text can stand on its own as a complete sentence. Your introductory text, This product is popular because, cannot.) One way to do it is this:

This product is popular because it is (1) inexpensive, (2) covered by a two-year warranty, and (3) made of environment-friendly material.

Unfortunately, there is possibly a problem here with parallel structure. The main issue is that (1) is much shorter than (2) and (3).

_{Some readers may also object on the grounds that (2) and (3) are passives, while (1) is an adjective. I don't think this is correct, strictly speaking. It seems to me that (2) and (3) can be interpreted as adjective phrases. To see this, note that if one replaces is by seems, the sentence is still arguably grammatical: there are attestted examples of constructions seems covered by (e.g. here) and seems made of (e.g. here, here, and here). The point is that seem cannot take a past participle as a complement, whereas it readily takes an adjective phrase as a complement (CGEL, p. 79). Thus (2) and (3) may be interpreted as adjective phrases. Nevertheless, perhaps there is indeed a tendency to interpret them as passives, which leads to a perceived tension with the adjective in (1), and this is enough to make the parallel structure a bit deficient.}

In any case, further rephrasing may be desirable. One way to do it:

This product is popular because of its (1) low price, (2) two-year warranty, and (3) environment-friendly material.

Alternatively, if you really want the list items to be complete sentences, then you should probably make the list vertical. In this case, you should also (1) rephrase the introductory text so that it is a complete sentence and (2) enumerate the list using just numerals followed by periods, as follows:

This product is popular for the following reasons:

1. It is inexpensive.
2. It has a two-year warranty.
3. It is made of environment-friendly material.

Discussion

There are several issues here:

1. Should the reasons be presented as an enumerated list?
   
2. If so, should the list be run-in or vertical?
   
3. How should the list be introduced? (Because is not the only option.)
   
4. In particular, should the introductory text end in a colon?
   
5. How should one enumerate the list elements?
   
6. What form should the list elements take? (Complete sentences or phrases?)
   
7. Should the list elements begin with a capital letter?
   
8. Should there be ending punctuation for the list elements?

First of all, in general,

Items in a list should consist of parallel elements. Unless introductory numerals or letters serve a purpose—to indicate the order in which tasks should be done, to suggest chronology or relative importance among the items, to facilitate text references, or, in a run-in list, to clearly separate the items—they may be omitted. Where similar lists are fairly close together, consistent treatment is essential.

Many of these things may apply to you; thus, CMOS would allow (though not require) you to enumerate the list.

Lists may be either run in to the text or set vertically. Short, simple lists are usually better run in, especially if the introductory text and the items in the list together form a sentence.

A bit of a mixed bag here: you have a fairly short and simple list, but the items in it do not together form a sentence. Indeed, in the next section, CMOS would seem to suggest that you should use a vertical list.

In the case of run-in lists, CMOS says the following:

If numerals or letters are used to mark the divisions in a run-in list, enclose them in parentheses. If letters are used, they are sometimes italicized (within roman parentheses; see 6.5). If the introductory material forms a grammatically complete sentence, a colon should precede the first parenthesis (see also 6.61, 6.64, 6.67). The items are separated by commas unless any of the items requires internal commas, in which case all the items will usually need to be separated by semicolons (see 6.60). When each item in a list consists of a complete sentence or several sentences, the list is best set vertically (see 6.130).

According to the final sentence, you are better off setting the list vertically. Also, note that in your case, the introductory material (This product is popular because) does not form a grammatically complete sentence, and so CMOS would advise against the colon.

The principle that a colon can only come after a grammatically complete sentence is emphasized by many (and probably most) other authorities, including Garner's Modern American Usage (p. 654), Skillin and Gay's Words into Type (p. 181), Strunk and White's The Elements of Style (p. 19), and Grammar Girl.

In the case of vertical lists, CMOS says this:

A vertical list is best introduced by a grammatically complete sentence, followed by a colon (but see 6.131). There are two basic types of lists: (1) unordered, in which the items are introduced by a bullet or other such marker or by nothing at all, and (2) ordered, in which items are introduced by numbers or letters. If the list is unordered, and unless the items consist of complete sentences, each item carries no end punctuation and each can usually begin lowercase (except for proper nouns). For lists whose items require more prominence, capitalization may instead be preferred; choose one approach and follow it consistently. If items run over to one or more new lines, the runover lines are usually assigned a hanging indent (see 2.11). (An alternative to indenting runover lines is to insert extra space between the items.)
...
If the items are numbered (i.e., the list is ordered; see also 6.127), a period follows the numeral. It is customary to capitalize items in a numbered list even if the items do not consist of complete sentences (but see 6.131). Closing punctuation is used only if items consist of complete sentences. For the use of roman numerals and letters, see 6.132.

CMOS gives the following example:

Compose three sentences:

1. To illustrate the use of commas in dates
2. To distinguish the use of semicolons from the use of periods
3. To illustrate the use of parentheses within dashes

Moreover,

In a numbered list, runover lines are aligned with the first word following the numeral; a tab usually separates the number from the text of the list (see also 2.21).

CMOS's example for this:

To change the date display from “31” to “1” on the day following the last day of a thirty-day month, the following steps are recommended:

1. Pull the stem out past the date-setting position to the time-setting position.
2. Make a mental note of the exact minute (but see step 4).
3. Turn the stem repeatedly in a clockwise direction through twenty-four hours.
4. If you are able to consult the correct time, adjust the minute hand accordingly and press the stem all the way in on the exact second. If you are not able to consult the correct time, settle on a minute or so past the time noted in step 2.