I think there's three big reasons that interest in Teach for America is so high: ease of entrance, salary, and prestige are probably the biggest. There are few jobs that will pay you $45,000 with just a bachelor's degree in anything but the hard sciences. I doubt that there is any overwhelming interest in education specifically, but there is always the idealistic urge to give back to the community.
I think prestige accounts for the interest in TFA over say, the various teaching fellows programs. I can say for myself the thing that attracted me were the first two reasons listed above.