Because they are usually cheaper. Most ladies know how to price themselves. If they are older and realize they are less desirable to look at, then they usually know to price themselves accordingly. It's simple economics law of supply and demand. If demand is high, price is high. Demand is low? Then the price is low.
Not every guy wants to spend a hundred or two, in just 10 minutes. Some just want to fuck for 5 minutes, so they figure they just find an older lady who's game for 60 bucks. For some guys, a pussy is a pussy.