Why "interesting" is sometimes pronounced as "intra-sting"

For the same reason "surprise" is frequently pronounced as "sah-prise": people sometimes take shortcuts if the meaning is still clear even with the mispronunciation. For more examples, see here, including this detailed explanation of the specific pronunciations of "interesting":

Interesting IN-tris-ting or IN-tur-uh-sting or IN-tur-ES-ting.

All three pronunciations are acceptable, though not so long ago only the second was considered cultivated while the first was considered British and the third was frowned upon by some authorities. One reason there are so many accepted pronunciations is that most educated speakers do not say interesting in exactly the same way every time. Slight, unconscious variation is natural in rapid and informal speech, and when a certain variation recurs often enough in educated speech, it usually becomes the norm.

The three-syllable IN-tris-ting is a victim of syncope (SING-kuh-pee), the loss or omission of a sound or syllable from the middle of a word as in FAM-lee for family and KUHMF-tur-buul for comfortable (which see). It is now probably the most commonly heard pronunciation in American speech. The noun and verb interest is also a victim of syncope and is usually pronounced in two syllables, IN-trist, although the older IN-tur-ist is still heard. The verbal adjective interested is often pronounced in four syllables, IN-tur-ES-tid, but the evidence of my ears says the three-syllable IN-tris-tid is more common.

The four-syllable variant IN-tur-uh-sting, once the preferred pronunciation, is now much less common than the syncopated IN-tris-ting. The somewhat overpronounced IN-tur-ES-ting never had great currency. Speakers who normally say IN-tris-ting will sometimes use it for emphasis or ironically, drawing out the syllables, as in the stock phrase very interesting.

It's a case of Nature imposing the law of necessity and sufficiency on us. The missing vowel in question is not necessary for comprehending the word/concpt, but the remaining ones are, and so we have the situation you noted in numerous cases. These cases, of course, are a stumbling block for ESL students, and since I am an ESL teacher, I compiled a list of some of them (all I could conveniently get on one page). Here is the article:

Minimal Words with a

Silent Internal Isolated Vowel (SIIV)

in American English

Non-example 1: “ate” – the “e” is silent, but not internal.

Non-example 2: “seat” – the “a” is silent and internal, but not isolated.

Non-example 3: “promised” – the “e” is silent, internal, and isolated, but the word is not minimal – removing the “d” does not change the root meaning.

Non-example 4: “lineman” – the “e” is silent, internal, and isolated, but the word is not minimal, being a compound word.

Note: A siiv might be variable, as in “temperament”, which can be pronounced “temprament” or “temperment”.

Examples:

1. average  “avrage”
2. bakery  “(bake)ry”
3. Barbara  “Barbra”
4. basically  “basiclly”
5. beverage  “bevrage”
6. business  “busness”
7. cabinet  “cabnet”
8. camera  “camra”
9. chocolate  “choclate”
10. circumference  “circumfrence”
11. comically  “comiclly”
12. comparable  “comprable”
13. difference  “diffrence”
14. different  “diffrent”
15. Dorothy  “Dorthy”
16. elementary  “elementry”
17. every  “evry”
18. infinitesimal  “infintesimal”
19. interest  “intrest”
20. laboratory  “labratory”
21. literature  “litrature”
22. logically  “logiclly”
23. Margaret  “Margret”
24. memory  “memry”
25. miserable  “misrable”
26. misery  “misrey”
27. musically  “musiclly”
28. mystery  “mystry”
29. omelet  “omlet”
30. opera  “opra”
31. phosphorus  “phosphrus”
32. preference  “prefrence”
33. realistically  “realisticlly”
34. reciprocally  “reciproclly”
35. restaurant  “restrant”
36. salary  “salry”
37. separable  “seprable”
38. separate  “seprate”
39. separative  “seprative”
40. several  “sevral”
41. slippery  “slippry”
42. sovereign  “sovreign”
43. specifically  “specificlly”
44. strategically  “strategiclly”
45. tactically  “tacticlly”
46. temperament  “temprament”/”temperment”
47. temperance  “temprance”
48. temperate  “temprate”
49. temperature  “temprature”
50. tragically  “tragiclly”

Missing out a weakly stressed vowel in a word (as the first e in interesting is) is common in spoken/vernacular English. It is done simply out of convenience, as it makes the word easier to speak. Call it laziness if you will.