Money

Well, I think in askfm's case it was the never ending barage of questionsm

It basically might be finances. And staff. We are used to use these services but we do not support or pay for the cost of simply running and maintaining them, so it is difficult to keep them running.

I don't think that both shutdowns are related, not that you were suggesting they were.

Maybe general purpose Q/A websites are too hard to make money off of. But Quora somehow knows how to stay strong after all of these years. That's more knowledge based Q/A but still.

Probably just the cost of hosting/bandwidth and/or the time and effort of development/maintenance and whatnot

Financial reasons. The ask.fm CEO looked at the retrospring closing notice and thought it wouldn't be sustainable to absorb such a big userbase into their platform. Retrospring is going to end soon, but our lords at least released the software so anyone can make a retrospring 2.

They take too much effort to run, admins don't get any money to run it and their users aren't grateful and constantly demand improvements or to ban people (or cause drama).