There's a common tendency in many sitcoms to abandon their
original premise when one character becomes more popular than
the others. Probably the best example is Family Matters, which
eventually just became a showcase for the antics of Steve Urkel.
In the case of The Big Bang Theory, that character is Sheldon
Cooper, the Aspergers-esque theoretical physicist. Sheldon has
become more and more the focus of the show, which just
illustrates the grating, obnoxious qualities of his character.
But the show wants to have it both ways, with you laughing at
Sheldon but also empathizing with him, and it just doesn't work.