By the time h.265 encoding was trying to gain traction, h.264 encoding speeds were very fast. The image improvement was negligible with the main benefit being smaller file sizes. For the average user, the increased encoding times did not justify that. The switch from MPEG-2 to h.264 had very noticeable quality improvements so it did make it worth while for the slower encodes until h.264 was locked and key code included in CPUs. It was similar to the adoption rates of DVD from VHS compared to Blu-ray from DVD.
That's a contradiction because quality improvement and file size improvement are just two sides of the same coin. You can't have a large quality improvement at the same bit rate without having a large file size reduction at the same quality.
The average user is a consumer of media, not doing encoding themselves. A one time cost for higher encoding to save bandwidth / storage space many times over is almost always going to make some amount of sense.
The real issue here is just a standard chicken-and-egg problem. To use a new codec, you need it to be supported in end user devices. To get it to be supported, you need to show demand... for a thing that nobody can use yet.
Another reason to get a new video codec standard finalized sooner is that hardware implementation and deployment in mass market consumer SOCs is glacially slow. On the software side, encoder and decoder performance tends to improve meaningfully in the first few years as optimization occurs. And those running large media distribution platforms prefer at least 12-18 months to evaluate and implement a new codec.
Perhaps: