I think a lot of it has to do with personality and flavor of the individual sites. The ones that don't delevope either fade away. Others go away for financial reasons or they just get tired of being hacked or spammed. I remember the beatings Skinartistry used to take from spammers and hackers and that was a good little site with a very distinct personality that was fun and supportive and it wasn't always about skinning.
I don't believe anyone thinks they can or that anyone wants to take anything away from WC. On the contrary, I think they see WC as a role model and just want to put their own spin or flavor on it because they can or because they might feel they can't here. (Just look at some of the negative comments in this post over something that hasn't even gotten off the ground and doesn't have a fraction of the financial support or backing that WC has)
I seem to recall (I don't feel like looking it up) that part of becoming a Master and/or Journeyman here was how much an individual contributed to the ENTIRE skinning community and specifically pointed out sites in general outside of WC. I think starting one to offer skinners and would be skinners options, help, tutorials, etc. etc. (where they may feel more comfortable in a smaller community without all the negative stuff that you find in any large community) certainly qualifies.
Plus, I just think it's brave and bold and daring and a hell of a lot of responsibility and the folks who make the effort should be commended for doing so and not discouraged or have their motives questioned.
The internet is too damn small and the skinning community is even smaller for there to be any damn animosity or BS.