Updates are delayed to give add-on developers time to update their add-ons to be compatible with Thunderbird 91 if changes are necessary.
[0] https://www.mozilla.org/en-US/security/advisories/mfsa2021-3...
And who can blame them really, no one pays them to work on these plugins. If you wanted to make sure your plugins worked before an update than you should go test them on a beta yourself.
Unfortunately delaying an update doesn't really solve the problem as developers still wait for the "its broken" reports before fixing things.