One of the reasons more people don't visit Sukhothai, aside from the lack of accommodations, is that the ancient capital isn't well connected to the other major tourist centers. It's too far from Bangkok or Chiang Mai to make a reasonable day-trip destination, and until recently, it didn't have an airport. The nearest train station is in the neighboring province of Phitsunalok, although train travel in Thailand isn't very appealing. So, for independent travelers, Sukhothai just hasn't been a very feasible option.
Flying to Sukhothai
Recently, "boutique" airline Bangkok Airways built one of their "boutique" airports in Sukhothai province. Alternatively, you can fly Thai Airways into Phitsanulok, which is only a short drive away from Sukhothai.