The Xel’Naga sought out to engineer two species: one with purity of essence; one with purity of form. Throughout the natural course of their growth, the two species would come together and merge. The Xel’Naga would be reborn in this perfect combination. They did this successfully for eons, and this time the Protoss and Zerg were chosen.
This is what the article said. What is it about the Protoss that is so great that for eons the Xel'Naga haven't managed to make before?

I'm also still wondering why they would continually repeat the cycle instead of just using the same species to replace their host bodies when they grew old. Unless they just did it for kicks, or wished to keep improving themselves every time. Which means that having done it for eons they would of had a pretty killer species to take host bodies from if that were the case.