The action scenes and involvement with weebs (from Dead Fantasy) set the foundation.
Then RT got involved, getting the "Nu-Geek" crowd involved, as well as early youtube users (the kind who watched RvB when it was new).
With the above you have a very vocal community.
The weebs discuss it- even if it's to say they don't like it. Some hate how aggressive weebs are about their opinion, so they look at the show almost out of spite.
> Christ that guys really got his tits twisted over this show. Wonder what it's really like.
The casual geek/dilettante geeks are into it so they can look "quirky and unique", and on top of that it's a new and unknown show, attracting hipsters or those with desires to be unique in liking something unknown they can boast they have knowledge about.
And the early youtubers are in into since their mentality is "It's free, and I'm doing it to burn time" so they're less concerned with quality (think of it like how Apps on Smartphones are now).
All of the above act like free advertising, so more people know it exists and checks it out.
As for how it got big in Japan- I haven't a fucking clue.
I know Japs like some western animation (capeshit and Disney), but I figured they liked hand-drawn over CGI. I also thought otaku hated western people trying to emulate anime/the community and especially trying to make their own. Maybe it tickled the fancy of casual anime fans in Japan, and the show had just the right waifus?
Now the above does risk more western SJWs or busybodies squirming their way into Japanese animation industries (Crunchyroll is respected, but they're oblivious the companies origins), but hopefully the free market and otaku rage will sort that.