I hate to say it, but I think it's just what the show has turned into after so many years. It's not as novel as it was when it first came on, and the original team has been broken apart, and I guess after so many years for some people the only draw of it now is their favorite character. At least that's what I think.
I kind of feel that way too. From seasons 1-5 I can watch just about any episode and be happy and love it, even if Nick isn't in it much. But these days I'll admit he's the main reason I watch. I don't think the show itself is what it used to be, at least not to me.