I think the main problem with Cradily is it's speed, and the fact that it cant handle basics. If you played something like ZRE against Cradily, Cradily just wouldn't be able to handle it at all.
Also, I think the fact that his attack doesn't do damage hurts a little. With something like rocklock, Ttar has a decent attack, and when you ATMROCK you hit their basics with sand damage; thus, you can build up a bit of damage over a couple of rocks. However Cradily's attack wont do damage over time if you keep doing it, and the other attacks it has makes ko'ing slow enough for you opponent to get the their evolutions back out.
I could see Cradily working with D.Amphy, but IMO, I think it's just too slow, and that's why it doesn't see play.
~Secretsmeargle