H.265 was always better from a technical perspective but that's not everything which factors into a video codec decision. H.264 was supported everywhere, including hardware support on most platforms. You could generate one file and have it work with great experience everywhere, whereas switching to anything else likely required adding additional tools to your workflow and trying to balance bandwidth savings against both client compatibility and performance — if you guess wrong on the latter case, users notice their fans coming on / battery draining in the best case and frames dropping in the worst cases.

Encoder maturity is also a big factor: when H.265 first came out, people were comparing the very mature tools like x264 to the first software encoders which might have had bugs which could affect quality and were definitely less polished. It especially takes time for people to polish those tools and develop good settings for various content types.