Why did it only evolve once in only one tiny group? Arthropods evolved several times into many diverse groups, only one linage for vertebrates...
In the Cambrian explosion many different types of arthropods were extant, but only two vertebrates are known, one of them had compound eyes and didn't go on, one of them was a tiny creature that could easily have gone extinct, if it had there would be no vertebrates, no other line was evolving in that direction that we know of but many external skeleton animals were present. it would have been a simple thing to have a world full of invertebrates but no vertebrates. As i said, internal skeletons might have gone on but vertebrates look like a bad bet...
I have to admit i am basing most of my argument on the Book, Wonderful Life by Gould... http://en.wikipedia.org/wiki/Wonderful_Life_(book)