A theory proposed by a professor at my school was:

The black death killed a lot of the people who grouped in churches and ignored the common sense precautions of staying away from people. This could be seen as a culling of what was largely the weak and foolish third of society.

Coincidentally, with fewer people to do the labor around and a generally smarter stock of people remaining, the upper classes were forced to pay more for services that keep the world running. There was a huge impact on labor markets, claimed the professor.

Thirdly, and likely related to all of this, the renaissance began.