The problem with having swords on Shift is that you need it in various sizes. And if you only offer 2.25u/2.75u most people would end up with only one sword anyway. Using 2.75u Shift is just wrong (imho) and ISO users would also only have one sword key. Thus, having that one sword on Enter makes more sense, I think.
Of course, you could have a 1.75u sword as well, but on 1.25u you could only fit a dagger at most. And this leads to even more keys and costs.