The problem is that Apple hadn't made a GM (Golden Master) release of iOS available to developers until yesterday. This means that developers have had little opportunity to submit updated apps that both fix issues caused by the new releases or to take advantage of the new features.
And it's not just iOS 14. Also affected are iPadOS, TvOS, and WatchOS. But it's iOS 14 that's the big one, and it could be live on tens, if not hundreds of millions of iPhones in a matter of days.
And developers know that, if there are problems, users won't be screaming at Apple. They'll be screaming at the developers.
Some developers are going as far as actively advising users who don't want to experience problems to hold off updating until they get a chance to get their ducks in a row.
And if you do find yourself hip-deep in said tarpit, remember, don't blame the developers. Blame Apple, which rushed this release out the door. Or blame yourself for thinking you could outsmart launch day bugs.