Because the notion that the content of everyone banned on Quora is unworthy of being read is a regressive and untenable notion, and Quora itself acknowledges that. Users get banned for any number of reasons. Some of the best damn content on this site has been produced by users who have subsequently been banned. A website that would choose to repress forever all the content of Michaelis Maus or Mary C. Gignilliat or Glenn Rocess or Bill Streifer or Taza Gul Kamran Khattak or Basil Fondu—to mention only people banned in the last week—is not a website I want any part of.
At this point, I would normally say “Downvote what you don’t want to see, and upvote what you do.” Curating your feed is how to stay sane on this site, and it does actually work. You’re reporting that your problem is with the Quora Digest, though, which you don’t have as much control over. While I was not that impressed by the Quora Digest myself when I gave up on it, “garbage questions from banned accounts” suggests that your digest email is full of troll questions. The Quora bots are dumb, but I’m surprised if they’re that dumb.
But the Quora Digest you receive is still customised to the preferences you evidence while logged in to Quora. I’d suggest suspending subscription to the digest for a few weeks, some enthusiastic downvoting and muting of topics and content you don’t want to know about, and resubscribing afterwards, and seeing if it makes any difference.