Interesting that you should say that 70# plus bows are hard to move. I don't see a lot even for sale :/
Five years ago, when last I was in the market for heavy ASLs, there were lots of 70# plus bows out there, but nowadays it seems like a famine for heavy longbows. I guess lots of the good ones are being held onto. I see LOTS of 45-55# bows for sale.
Maybe I'm just getting (un)lucky, over the last couple months. You know what happens when you start looking for something specific...
I suspect a lot of Trad archers hold onto their good ones, heavy or lighter, and are getting older, so all of the demand is for the lighter stuff.