No, I can't explain it. It was just an arbitrary rule I was taught as a kid - and that was quite some time ago! There was never a reason given for it, just one of many rules we were supposed to remember.

A lot of other people were taught the same rule, but hopefully they've realized as I finally did that it just doesn't matter.

Here are some interesting discussions from both sides of the fence:

https://www.google.com/search?q=due+to+because+of

Joseph Emonds argued in the paper "Grammatically Deviant Prestige Constructions" that the underlying reason for some of these arbitrary distinctions was as a social class signifier: if you used "correct" grammar it helped indicate to others that you were a member of a more prestigious class:

http://fine.me.uk/emonds