They explained their decision I think it was in late 2017 or early 2018. Reducing bitrate of lower encoder time are not their priority. Even at the same bitrate x264 still beats x265 in terms of video quality. There are some problematic scene x265 just doesn't do well.

If they could reduce bitrate while getting the same quality, they would. If they could get even better quality while having the same bitrate, they would do it too. The problem is at their quality level requirement x264 is still king. And as of late 2018 that still seems to be the case. ( Tuning Encoder is a god damn insane job )