Not getting desirable results
Economic crisis
Getting bored of it

“It has been clear for some time that motorsport cannot ignore the world economic crisis,” the FIA said in a statement. “Car manufacturers cannot be expected to continue to pour large sums of money into Formula One when their survival depends on redundancies, plant closures and the support [...]

With 1 victory under their belt over 3 years, the Hinwil based team (BMW Sauber) has just confirm their exit on Formula 1. They will leave at the end of the season. BMW Motorsport Director Mario Theissen says:
“Of course, we, the employees in Hinwil and Munich, would all have liked to continue this ambitious campaign [...]