Because winning championships or competing doesn't necessarily equate to dollars. Case in point, the Knicks can't win a ing game but they are one of the most profitable organizations in the NBA. The Spurs have won 50 games a season for 16 or so years and they are down the list, behind teams that haven't won .
Why? Because revenue is based on market size and knowing your demographics. LA fans are Asian and Mexican mostly, with a bunch of blacks scattered throughout the country who bandwagon during the good years. The money though comes from people who don't know a lot about the game, they know who Kobe is though. They want to see Kobe play. MJ would sell out road games in arenas that couldn't sell out games otherwise. Kobe has the same effect, but not because he's a great player. He has that effect because he's a household name. You have that, you don't part with it unless you're in a market that doesn't allow you to capitalize on it. LA can capitalize on it. So it was a smart business move.
There's a facade in the NBA and in most professional sports that the game is about winning, but winning to you and me means holding a trophy. To them it often means ticket and merchandise sales.