Same reason anything that gets popular gets so much hate:
a) fanbase & people's reactions
b) trolls
c) overhyped
Personally, I don't really get all the hate for these series. I've read a fair amount of Naruto and watched all of Bleach that's currently out. Neither are stunning, they're both decent in their own right and I enjoy watching/reading each of them, they're good series. Not great, not terrible, but good. I haven't seen/read any One Piece as it doesn't particularly appeal to me, but I'd imagine it's the same sort of scenario. Now, obviously people will have favourites and there'll be preferences, but generally I think they're all (probably, for One Piece) good series in their own rights. There are better things out there, but there are far worse too.
But, if something gets popular people will hate it. It's not just Naruto, Bleach & One Piece, take Haruhi, Lucky Star, Death Note, Code Geass, DragonBall. I can guarantee if something's popular some people will hate it unnecessarily.. They may hate it because so many people like it when they don't think it deserves it, they may hate it because people keep going on about it and they're sick of it, or they may hate it just because they can.