Why does NPR have such a bias toward "progressive" coverage?

Because conservatives don't say much of anything that's true.

I don't know. I've never personally listened to NPR. Though it would probably be because that's the general trend of Western culture in the last few centuries. Rights have been expanded to different religious groups, races, women, the abolition of slavery, and so on. These have in turn led to advances in Western societies that led us to where we are now, basically leading the world in terms of wealth and technology. Particularly economic growth has be stimulated by the west's progressive tendencies combined with relatively free markets.

Some are no longer pleased with this trend though. In particular the recent extension of marriage rights to homosexuals and the acceptance of Islamic individuals has piqued debate. It seems unlikely that the trend will be bucked however, at least in the long term. In an increasingly wealth-centric society profit holds much influence. We even refer to it as "the bottom line".

They were to be the answer to Rush Limbaugh ... but failed miserably

It's all about the money