In general, subordinating conjunctions become part of the subordinated clause they create, which means the position of the subordinated clause (including the conjunction) in the sentence relative to the other (main) clause can change. Coordinating conjunctions, however, must remain between the two clauses (or whatever elements).

In the case of "for" and "because", any instance of "for" can be replaced (on the surface) with "because" and it will still read properly. But not every instance of "because" can be replaced with "for":

*"**Because** I have a wife of my own, I
   cannot marry you."*

is fine, while this is not:

*"**For** I have a wife of my own, I cannot marry you."*

"For" must always come between the two clauses.