Expiring music rights, exclusive deals, and shady developer practices all contribute.
The issue seems to be tied to publishers, not developers. Many of the removed games were linked to a sole Russian publisher, Dagestan Technology, that appeared to be operating under a number of different names.
The peril of DRM can raise its ugly head years down the road. A good example is Tron: Evolution, which recently became unplayable after nine years because its publisher, Disney Interactive, stopped subscribing to SecuROM.
The Epic Games Store arrived with a splash, luring devs to its marketplace with a better revenue cut than Valve offers. It doesn't hurt that millions use Epic's launcher to play Fortnite, either.
Grid, Dirt 3, F1 2013, Tony Hawk Pro Skater HD, and numerous other games have vanished from Steam and other digital marketplaces due to music licenses expiring.
A good example? Dark Souls: Remastered replaced Dark Souls: Prepare to Die on Steam back in 2018. You can no longer buy the older version of the game on there.
Batman: Arkham Knight is maybe the most well-known example of this. After launching on PC, the port was widely criticized due to performance issues, after which publisher Warner Bros. suspended digital sales.
Censors in China have banned Winnie the Pooh due to memes that used pictures of the fictional teddy bear to make fun of Chinese president Xi Jinping. Yeah, the world we live in is that stupid.