DaFuq? What does anime piracy have to do with shit, you like the majority eats Pokki and watches...sheeit, can't even name any of that crap for a comparison...uhhh...Battletech?
Again look at the formats that HAVE taken off...what do they have in common? EVERYBODY CAN USE IT, sure there are tools that can use CLI, fuck I'm sure you can encode MP3s in CLI...but nobody give a fuuuck, the majority are using easy GUIs...of which jack and shit exists for WebM. Fuck even the last Handbrake I checked had HEVC support...no support for WebM.
"Everybody can use it" isn't about creating, it's about consuming. If you want your format to take off you need it supported on consumer electronics devices. Again -- look at pirates. You know why h264 took so long to take off? It's because there weren't a whole lot of stand-alone devices that supported it several years back. Same with MKV container format. Been around for quite awhile, not supportted commercially. This is why the whole live action pirating community continued to put out Divx/AVI files like it was still 2002 until a few years ago. It was because there was a large installed base of DivX DVD players and first gen consumer electronics with PC file playback that wouldn't support MKV containers, and anything more than Main Profile h264.
The anime fansubbers didn't care about standalone players because once they stopped hardsubbing they required a PC for playback anyway (for SSA subtitle rendering). This allowed them to keep following new advances in encoding -- they weren't having to maintain backwards compatibility with people still running XBMC on actual original XBoxes. So they changed to h264/MKV back in 2005 and a few years back started doing shit in 10-bit h264. But they still do it in CLI, and the high-quality live action pirates, too. Nobody is wailing for a GUI but people too lazy to learn to type.
WebM ain't taking off because h264 has already established itself as a standard outside a web browser. It's the convenience to the users that counts.