I'd assume because DASH is generally a newer spec, and HLS was driven by mobile usage
Don't confuse DASH and HTML5. DASH was announced VERY shortly after HLS (within the same year). And both of these predate modern video on the web. HTML5 video does not play DASH anyway. MSE can play the same binary format as DASH, but that format was developed by microsoft for smooth streaming first.
Of course Twitch should support HTML5 video though, no idea why they aren't.