My hunch is that many of these exchanges were part of the sudden influx of exchanges in 2016-2018, the time when thousands and thousands of tokens flocked the crypto market. Now, with so many altcoins dead and trading volumes of those which remained are terribly falling down, profits are also decreasing, definitely touching rock bottom to a lot.
Or perhaps the crypto market is too small to accommodate all of them, considering that the largest pieces of the pie are only divided among few exchange monopolies.
In other words, it must have something to do with profit, at least to many of them, which is not mentioned in the article. Operating and maintaining a crypto exchange might not be a profitable undertaking anymore that's why they are forced to voluntarily shut down. This must also explain the very high count of abandoned exchanges, those MIA. They wouldn't have been abandoned in the first place had they been providing juicy income to the owners.
this would be my conclusion also.
there are some people who think just because something is profitable they can also make profit if they enter that scene, but since they have no understanding of the business aspects of it, have no experience in running a successful business, are too lazy to work on it and improve it,... they simply fail. we even saw some discussion on bitcointalk with someone who was trying to build an exchange and had no understanding of the business aspects!
we see the same thing in different fields too. for example a couple of years ago there was "dice site fever" and within a couple of months we saw dozens of new dice sites be created and 99% of them failed and died.
there is also another problem in the exchange business which is people stick to the big exchanges even if they are the shittiest and shadiest services. for now the Binance exchange has taken over the altcoin market and practically controls it so there is no room for an alternative which is another reason why any new poorly designed exchange fails.