The explanation begins with the grammatical fact that a coordinate conjunction — “and,” “but,” “or,” “nor” — must link grammatically EQUAL parts. It cannot link an independent clause to a dependent clause. Remember that it can never be “He is tall and Bob.”
So if this is true — and it is — we have to look at this construction:
…I know that he was tall and that he was heavy.
…I know that he was tall and he was heavy.
In the second sentence the “and” has to either be linking “that he was tall” and “[that] he was heavy.” OR it has to link “I know” and “he was heavy.” It makes more sense to assume that there are two things that “I know” and that the word “that” is understood in the second clause.
…She left him because he drank and because he ran around.
…She left him because he drank and he ran around.
It makes sense that she left him for two reasons: drinking and running around. The “and” is linking the two dependent clauses with the word “because” understood in front of the second one.
Yes, it looks like an independent clause; however, it is our tendency to cut things down after the first utterance when we are using similar patterns.
…She left him because he drank, he ran around, and he didn’t work.
There are three reasons that she left him. The “because” is “understood” in the second two clauses. So the commas work as these are not separate sentences.
I hope this helps.