If we want to use due to as an adjective, we would not want a due to in there.

X is due to Y.
This could be acceptable.

X is $20 billion due to Y.
This is not acceptable. We're not talking about either X or $20 billion separately.

Let me know if I've overlooked something. I'd be happy to know about it.

Look at this as a meaning issue rather than something to do with grammar. The because of is incorrect because it does not give us the right meaning. We are not looking to provide the reason for something here. Instead, we're looking to provide, loosely speaking, a category.

Last year, we lost $20,000 because of bad marketing.
The bad marketing bit identifies the cause of the loss, but not the type of the loss ("category").

Last year, bad marketing cost us $20,000 in lost revenue.
Last year, bad marketing cost us $20,000 in lost profits.
Last year, bad marketing cost us $20,000 in lost goodwill.

You can see here that lost revenue/profits/goodwill is not something that tells us what the problem was (the problem was bad marketing). Instead, it's telling us what that $20,000 was "made of". In other words, the possibility that bad marketing actually led to a much larger overall loss exists. That is, maybe the overall loss was $60,000 (20 in R + 20 in P + 20 in G). So we could even combine those sentences and say something like this:

Last year, bad marketing cost us $5,000 in lost revenue, $10,000 in lost profits, and $20,000 in lost goodwill.