Is "because" a preposition & "for" a subordinator?
Which CGEL? "The Cambridge Grammar of the English Language” (Quirk et al) - or "The Comprehensive Grammar of the English Language” (Huddleston et al) – (I suspect Huddleston - I don't think anyone else would call "because" a preposition.)
Consider the nuances: 1. I bought the book because I wanted to read it. -> caused by my wanting to read it
I bought the book for I wanted to read it. -> for [the reason of] my wanting to read it.
I bought the book as I wanted to read it. -> [in the manner/circumstances of] my wanting to read it.
I bought the book when I wanted to read it. -> [at the time of] my wanting to read it.
I bought the book in Paris where I wanted to read it. -> [at the place of] my wanting to read it.
Etc.
You will see that “because” encompasses all types of “causes”, e.g. reason; purpose; manner; environment; time, etc. but “for” tends to be more specific
On usage, “The New Fowler's Modern English Usage” (Revised third edition 1998) explains
for.
1 For is sometimes used as a coordinating conjunction, i.e. one that connects two independent sentences (see 3 below), but it should be said first that its most frequent use (apart from its employment as a preposition) is to join a sentence and a non-finite clause, esp. one of the type 'sentence +for + noun (phrase) + to-infinitive': he waited for the lock to click; it is time for their legal rights to be clarified. In such constructions the negative particle is normally placed before the to of the infinitive: it would be more sensible for the code not to be enacted as law.
2 Unlike some other coordinating conjunctions its position in the sentence is sequentially fixed (though even a century ago it was not), i.e. it cannot normally be placed at the beginning of a sentence. Its function is to introduce the ground or reason for something previously stated: *he picked his way down carefully, step by step, for the steps were narrow—*G. Greene, 1988 (for the steps were narrow could not have been placed before he picked his way down, etc.). In this respect it differs markedly from other coordinating conjunctions, e.g. because, since, which suffer from no such restriction.