Powerboats have very expensive engines. Don't know enough about them to make an educated guess, but figure the cost of the engine must be half the cost of the boat (maybe more ??).
Maintaining them (the engines) is a lot of work, costs a lot of money. So I think that the amount of money invested in a powerboat is quite a bit greater on a foot-by-foot basis, both at initial purchase, and for ongoing maintenance.
Powerboating is easier than sailing. You turn a key and go, don't have to hoist, pull, tack, gibe, reef, hike, etc. The first few times out sailing on your own can be frustrating, if you are not comfortable on the water to begin with, it can be scary.
So on one hand the (power)boats cost more to buy and maintain hence owners have made a bigger investment in them than a comparably sized sailboat, and on the other hand, they are easier to deal with than sailboats.
I think the former accounts for their higher price, and the latter factor accounts for their popularity.