Because he was losing 10 million dollars a year and wanted a shiny new stadium like the rest of the Seattle franchises have gotten.
He was losing money, or at least he wasn't making much money. Even a renovated Key Arena is not bringing in that lucrative. I think he was frustrated by the new arena deal failing, and he wasn't willing to move the team himself. It's a lot easier to let someone else blackmail your home city and possibly move the team you idolized as a child. Clay Bennett also offered him way more than the Sonics were worth. Schultz really would have been a fool not to sell.
What's happening to Sonics fans is exactly what happened to Oilers fans. If the Sonics were a playoff team, this wouldn't be happening. Moving a franchise with that kind of history would be a terrible move. The fans in Seattle deserve better.
The Seattle fans have no one to blame but Howard Schultz who prefered to sell the Sonics to out of state owner and sell it fast. Now, the Seattle fans see Schultz as the saviour of the Sonics? Poor souls. Some more opinions