Users don't like to pay much for a thing that will be discontinued after a few years and need to sell it cheap just to upgrade to the new mkII and only to find in a couple of years they have to do it again.
Depends on the unit. The Axe-Fx III was so popular, users had to join a months long waiting list just to get one initially. But the Axe-Fx II had been on the market for over 7 years.