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One case I decided against it was when I needed to access the new postgres server (which I initially planned to be v13) from an old machine (with a legacy distro with v9.6). v13 introduces a new SCRAM-SHA-256 password hashing method by default and only libpq10 and newer supports this method. For some reason I couldn't or didn't want to neither rehash the passwords nor upgrade the client, so I remained on a lower version. Certainly not unsolvable, but I didn't have the time to spend on a proper fix.

What situations would make people unable to upgrade to 12+?

In my personal experience: the organisation doesn't invest in a storage solution that offers snapshots/COW (e.g. ZFS or SAN or whatever). Then they wait to upgrade until their disks reach a usage that the upgrade has to be done in-place. Then they become like rabbits in the headlights and never upgrade.

Maybe it doesn't get prioritized until it's important. I know PG upgrades are pretty straightforward, but sometimes people don't want to touch something running well.

That said, given the performance implications, if someone wants to use PG as a warehouse upgrading to 12 is a no-brainer.