I wish that modders would use your site, as its so much easier having one site to look for mods and than a whole bunch.
To be fair that site is pretty new and some of the others have been around for many years. By your logic then, it's bad that Holds of Skyrim even started up because we already had Nexus, or even older, PES.

Alternately, you are suggesting that since Holds of Skyrim came out all others need to bow out and just encourage everyone to use that instead.
I don't mind variety; everyone does things there own way, and has their own style. The only thing that I found bothersome was when people decide to create too many smaller fan websites and it was the sole place they hosted their mods, but then over time they lose interest and the site ends up left to decay with no support or vanishes altogether and it's the only place they decided to host their mod, so now it's gone forever into oblivion (pun intended).