First of all, everyone who posted so far needs to work on their reading comprehension. OP asked why wouldn't Bosh SIGN with the Kings, not why don't we trade for him.
This is actually something I've been thinking a lot about. Prior to this season, the accepted reasoning around here was that we could only build through trades and the draft because no one, especially a super star, was going to willingly sign in a small market with a rebuilding team. Evans changes all that. This is no longer a rebuilding team. This is a team pushing hard for the edge of the playoffs, suddenly loaded with talent. We've got one of the best PGs in the league, one of the top scoring guards in the league, and a stable of extremely talented and versatile young players to compliment any superstar player.
At this point, signing with Sac might be from a purely basketball standpoint one of the best moves any superstar could make. LeBron has proven that you can be a global icon in a small market. Don't doubt that playing for the Kings would get him the 'King of California' label- he would be in the capital, after all

.
What other team that could potentially grab a superstar has as much talent as the Kings? Miami is close. Wade is great, a notch above Evans, and after that what, Beasley? Kings have a better team top-to-bottom.
Only problem is, we would need to make some trades to get a little extra cap room. Maybe it could be done in the off-season for a trade exemption, not sure how that works. But there should be no doubt that Sacramento, one of the most talented young teams in the league, should be on the radar of any superstar looking to change teams.