Why aren't more releases x265?

Nearing the filling of my 14.5TB hard drive and wanting to wait a bit longer before shelling out for a 60TB raid array, I've been trying to replace as many x264 releases in my collection with x265 releases of equivalent quality. While popular movies are usually available in x265, less popular ones and TV shows usually have fewer x265 options available, with low quality MeGusta encodes often being the only x265 option.

While x265 playback is more demanding than x264 playback, its compatibility is much closer to x264 than the new x266 codec. Is there a reason many release groups still opt for x264 over x265?