The situation in bio seems dire, but in physics our postdocs are typically $50k+ (obviously depends on where).
1. NIH stipend levels are pretty low. When you talk to people on NSF or DoD funding, they're like "You pay your postdocs what!?"
2. There are fewer "eject" options. Physics, for example, has a long and established pipeline into finance. The group I'm with does lots of work on complex networks - that has applications in tech. Bio work, for the most part, leaves you qualified to do bio work. So there's both less salary pressure for "We have to pay you or you'll go work for tech instead" and the feeling of your education being a sunk cost.