I don't like the turn raise as what do I represent? If I had a set, why would I raise? So now it looks like I just have draw. Maybe AK of spades makes sense. But even with that why would I raise? So simply raising the turn I cannot see any of my range that I would do this with.
So I simply thought that if I bluff it has to be on the river, as also this is where very few players bluff and thus my line would be more credible.