That's pretty easy to answer: His confidence is in the toilet right now. It's a snowball thing. He's had some bad games, and then he reads what is being written about him (including here. You can bet he reads every word written about him in these forums), and this makes him think too much about every shot he is taking. And that is never a good thing.
I've said this several times before and I'll say it again. Paul Westphal knows exactly what he's doing. He has been starting Noc despite his shooting slump because he didn't want to crush his confidence and lose him for the rest of the season. He is doing the same with Hawes for that reason, and also because he doesn't really have anyone to replace him. Granted, Spencer's game has been bad lately, but while we all know that he will not become an allstar this season, he will also not be as bad as he has been for the rest of the season. He will gradually bounce back, as long as Westphal continues to work with him and show him that he has faith in him despite this slump. When he's back in decent form, that's going to be another big step in the Kings becoming a real force in the NBA.