BXs are for sale because nobody upgrades to a BX.
Respectfully, I disagree. I upgraded to a BX (from a ride on mower). Lots of people upgrade to a BX, and lots of people would love to have a BX. Certainly in my part of the world they're quite hard to come by, and when they're for sale at a sensible price they get snapped up. Some people though are trying to sell a 3 year old machine for 5% less than new, and those don't sell well.
When I sold mine, which had been well abused and was quite old, I listed it and an hour later a guy messaged me and asked if he could buy it sight unseen. I explained to him it was pretty beaten up, but he wanted it anyway. I took his money and said obviously I put too low a price on it. He said no, the price was about what they sell for, it's just he had been in the habit of thinking about it too much, and so they'd come up and while he dithered it would sell. So he jumped on it.
I still believe that there are lots of BXs for sale for the simple reason that there are lots of BXes, it's one of the best selling tractors around. Even if people turn over a tractor once every 10 years, if they're selling a boat load of them new, then a boat load of them must eventually turn up on the used market.