With the exception of microniches (redhead twinks that like feet or something) I doubt it's a lack of models. More likely, it's laziness on the part of the webmaster, or s/he is either doing the model, is friends with the model, etc.

We will use a model repeatedly when the model is popular, but even so, we try to be careful to not saturate any one model, and not shit out the content in rapid succession. That's one of the reasons that I think Sammy Case and a few other twink models "burned out" so fast... they worked with a ton of sites who shit out a ton of content, and the audience got tired of the model.

I believe that can happen to a lesser extent with models that work for just one studio... you might like so-and-so from somebigcollegejocksite.com but if there are 25 scenes of that model, he is all over the tour and the ads and banners... people will get tired of him.