There are several reasons:
a) Kodi no longer supports OMXplayer and MMAL decoding as part of the general move to drop all proprietary codecs to focus on modern kernel standards (GBM/V4L2).
b) The Pi graphics pipeline in the newer kernel used by LE10 has been rewritten around modern kernel standards (GBM/V4L2) so "assisted" decoding would need to be reimplemented from scratch in both kernel and ffmpeg. The original patches amount to 100,000 lines of code and authoring that (and then maintaining is downstream) is a large effort.
c) Only a couple of distros (mainly LE and OSMC) have been daft enough to allow 100,000 LOC patches into their codebases so the audience that would benefit from a large reimplementation effort in 'B' is small.
d) RPi4 and up support hardware HEVC decoding (although to be clear, forcing hardware upgrades on users has never been a goal). It does mean over time the issues reduces in size along with the RPi2/3 user population.
e) Nobody is forcing users to update from LE9.2 which continues to have the feature.