The reason is that it's harder for them to rent those specialty classes and there are far fewer of them. For example, let's say you reserve a Corvette Convertible, then you decide you don't need the rental, but don't cancel and no-show. Now they're stuck with this expensive car on the rental lot waiting for someone who doesn't show. After a while (probably 12 hours or more), they'll assume you're not coming and maybe try to rent it as an upgrade to someone else, but they'll probably have to take far less for it than you reserved it for. If they have your credit card info on file, they can charge you a penalty for the no-show.

Contrast that with a reservation for say a Standard SUV. They probably have hundreds of those in the fleet, and if an extra few end up sticking around because of no-shows, they'll just give it as a free upgrade to someone who booked an Intermediate SUV or a car. It's much less of a big deal from an operational/revenue perspective.