Guideline: Use because or since when you might otherwise say "the reason being." Use as when you would otherwise use "during."
E.g.: I didn't hear any of the commotion that went on last night because [the reason being] I was fast asleep.
I didn't get a chance to see any of the finalists as [during the time] they were marching by very quickly.
Different from: I didn't get a chance to see any of the finalists because [the reason being] they were marching by very quickly.
Source: ProofreadNOW