I read everything as far as I know but I got sidetracked at least a few times so if someone has already mentioned this...
I assume a lot of chronic pain is caused by injuries and therefore not related to anything genetic. But if there are genetic reasons for chronic pain I can see two main reasons why so many people would suffer from it.
Firstly nowadays people can often be treated for conditions that may have been fatal way back when. Essentially, modern technology has allowed humans to circumvent nature killing off the weak and sick because the weak and sick can now be cured or managed so that they can enjoy an otherwise normal life.
Secondly the pain might not be severe enough (at least earlier on in life) that it would avoid the sufferer from having children.