It's a well known fact that
reality is unrealistic. It may very well be realistic for the Protoss to have skull shapes that are as different as Aldaris's and Tassadar's were from Fenix's and Artanis's (heck, maybe the differences in human skulls just happened to be miniscule, compared to other life forms in the universe)... but that doesn't mean that it's in the best interests of a science fiction work to pursue that. If the bounds of what does and does not look Protoss are not as firmly established as they have been going into WOL, it's a given that over the years we're going to get a lot of art that will push those boundaries and make us, ESPECIALLY us, with our rose-colored glasses and all, wince and say "What are you doing Blizzard?! You totally ruined the Protoss! They never looked like (mushrooms/goats/light fixtures/Arnold Schwarzenegger/wimps/whatever) before!"
The thing is SC1 doesn't present a single perfect answer to what looks Protoss. Back then we accepted it all on faith, each new image was a lesson that Protoss can indeed look like this, even though it's different from all the other ones we've seen before. SC2 does not have the buffer of being that first entry in a new franchise... we're not going to be that lenient. If Blizz came in and told us "oh and by the way Protoss can also look like this" and "this" was different for every new Protoss they introduced, I'm willing to bet the outcry would be a hundred times louder than what we had over the teleportation-upon-death crisis.