Never realized that it's quite that popular on pixiv.
I suppose the characters tend to be visually very distinctive and the whole thing appeals to my inner chuuni, so it stands to reason I'm not the only one.
And I don't think you should underestimate how much Fate had already established itself before FGO. VNs, actual games, an anime series or three, LNs...
Most other gacha franchises had to start from scratch and build up their clientele from scratch as well.