Because of his stance on health care. While Obama's plan would be a slight improvement over the current situation (in the short term), McCain's plan would have a much greater effect on the cost of health care because it attacks the source of the high costs...removal of consumer choice (competition) from the system. Obama's plan may be more efficient than the current system from a bureaucratic standpoint (yes, I realize that's a bit of an oxymoron), but it doesn't attack the fundamental problem, so it doesn't solve the problem of Americans spending a larger percentage of their income on health care than any other nation in the world.
The 'free market' is not the cause of the problem. The US hasn't had a 'free market' with health care since WWII. What we've had is a bureaucratic black hole.