CGEL included a wide variety in the category of preposition (but not "BECAUSE"). But some people say that 'for' can be a subordinating conjunction (subordinator) as 'meaningless marker of subordination' while 'because' is a preposition with a distinct meaning, which can take a subordinate clause as prepositional complement.
"I bought the book because I wanted to read it."
Here, 'because' is a preposition introducing a subordinate clause "because I wanted to read it." (I doubt it)
We can write this sentence using the subordinator 'for' :
"I bought the book for reading it". ("reading it" : gerund-participle non-finite clause).
Is 'because' a preposition or a subordinator? Can we write the sentence with the subordinator 'for' introducing a finite subordinate clause?
(Edit : "Reason clauses are most commonly introduced by the subordinators 'because' and 'since'. . . . " (CGEL, Quirk, Greenbaum, Leech Svartvik, Page : 1104.)