The plots got dumber and dumber just to suit Urkel becoming a star.
Two words. Transformation Box.
That's when I gave up for good. I could deal with him becoming "Stefan Urquelle", but when he turned into Elvis or Bruce Lee, it got beyond ridiculous. However, I do wonder how long the show would've ran had Urkel either remained a) an occasional character, or b) just a clumsy nerd who didn't make goofy crap like transformation chambers.
He's playing House's prison drug supplier this year, I see. That'll be interesting.
A year or so after
Family Matters was canceled, he did a UPN sitcom called
Grown-Ups with Soleil Moon-Frye. I remember it being pretty funny, but then again, my tastes were quite odd at 17. :-P
My guess is that it was too soon after playing Urkel for folks to see him in a completely different role...