This is a great point and I want to highlight it a bit.
I may not have played much of 4e, but what I did play supports this idea. A defender and their contribution to the party could not be fully matched by another role. Now, a lot of class did have a prime and secondary role. I think Paladins, going off memory, were Defenders who had a bit of Leader mixed in.
Now, I'd have to go and do much more in-depth research than I'm willing to do, but a part of me suspects that all Divine characters shared a similar focus. There was something that a divine Defender could do, that matched the idea of Divine characters, that a Primal Defender couldn't. I'd have to dig far deeper into the abilities and categorize them, but I suspect that is the case.