The reason quorum exists is to make sure enough people are looking at a proposal. And the whole point of Abstain is to let community members say, “I read this but I’m not sure what’s best, I’m trusting others on this one.”
Removing Abstain from quorum would give voters who don’t have a stance on the matter two options: either make their vote irrelevant and feel discouraged from participating, or go to someone they trust and just copy their vote.
One clear example of why this logic doesn’t hold: imagine a community with 100 members where only 3 have technical expertise, and there is a vote that requires that expertise. If 80 voters abstain, which means “I am looking at it and I do not know what is best,” and 3 vote yes, no, or whatever, would you really say this was not a strong proposal, whatever that means?