Not only because of the population but politicians would have a hard time justifying it when they openly complain about the same thing in other countries. Not that it has ever stopped them, but it'd be harder I think.

The US still projects an image of freedom to the rest of the world. It's certainly not idolized as much as a few decades ago but, when compared to other powerful countries, the general population still maintains a good perception of it (at least in the Western world?).

Anyway, a national firwall would create roadblocks for business and money talks.