Because traits were a way for them to give you skills early without having them be too powerful for your level at the time. It was also a way to make potential cross-class skills be more powerful on the class that they originally come from than they would be on other classes. Since the level 51-60 skills hit within just a couple of levels of each other, and can't be cross-classed, there was no reason to limit their power from the get go and upgrade them to their "actual" level with traits later.....