It wasn't that belts were too expensive. As demonstrated in this historical video of Prince Albert's wardrobe, wealthy upper class men also wore suspenders at the time.

The issue was that trousers in the 18th and 19th centuries were high-waisted and belts wouldn't have helped keep them up, especially because trousers weren't made with belt loops.

Trousers became lower waisted and began to made with belt loops in the 1920s.