That’s because H.265 is patent encumbered. Firefox doesn’t support H.265 at all and Chrome only supports it if the hardware does. In order to support accepting H.265 input from streamers, Twitch would basically have to pony up the compute resources for full-res realtime transcoding for every H.265 stream to H.264 – either that or put up with a lot of bad press surrounding people not being able to stream at full res anymore.