Because most people are saying "he's no fun because he's so OP and hard to beat". Everything after "because" is a L2P issue, and means the original claim, that he's "no fun", holds no merit.
If you mean he's little fun to play because he's so static, I'd have to agree; I thought I'd play Bastion a lot more than I ended up playing him. But I also tend to avoid a bunch of heroes for similar reasons. They're "not fun" to me, and I recognize that's an entirely subjective preference that doesn't reflect on the hero mechanically.
And that's the point; either you have an objective basis for the claim, which is where we get into balance claims, or you don't, and you have to accept that it's just your preference, and it isn't shared as broadly as you think.