Why not have PBS stand on its own without taxpayer support?

Public Television has some very good shows and some very mediocre shows. But why should they be subsidized by taxpayers?

PBS is largely funded by the Corporation for Public Broadcasting which is in turn receives 15% to 20% of its annual operating revenue from the US Government and 25% to 29% from State and local taxes.

When it was founded in 1970, PBS took over from its predecessor the National Educational Television. At that time there were few alternatives to CBS, NBC & ABC. But the world has changed. Now we have CNN, Fox, A&E, the History Channel & the Discovery Channel to name a few alternatives.

For some reason the fact that the PBS stations are largely non-profit gives it a free pass. Why not have PBS stand on its own without taxpayer support?