>>3968410Two reasons I think. First it's Fubuki himself who has a cool (and long-haired) design and this sort of chill, almost aloof personality that fujos tend to be into. Secondly, he is a tragic character turning villain halfway in and many like that too. It simply provides tons of potential for fanlore since it gives him additional layers to make him more complex.
Also, fanwork is often the result of cope (albeit in a good way). It's common to see fans being more obsessed with characters that get bad endings than winning heroes because they feel like they have to set things right and create a better outcome for them.
It isn't even any of my main pairings (I am more of a team Shinano fag) but I think I understand why it's so popular and I honestly like it too. Especially the art it spawns. Fubuki is getting among the best fanart of ngwk IMO.