Why are accommodations so expensive and require a minimum stay?

I was planning a day trip to a city about 250 miles away, but would require an overnight stay. I inquired about accommodation there. Some places require a minimum 3-night stay. One B&B said their rate was $300 and I figured that was for a week - WRONG! That was just for one night! When my dad was alive and the family traveled, it was considered a lot to charge $20 a night at the Holiday Inn. Where do they get off charging these exorbitant rates?