Since it's "sold out" they know what kind of money they are working with. Adding acts after the fact would only build more hype especially for next year. In a business sense I think it's smart to add acts. The line-up isn't as strong as last year's but with TWO weekends now... the money is there. They have room to add even if it's not an identical line-up.
I only put "sold out" in quote marks due to the whole credit card / error / decline debacle that I (and a few hundred people?) were victim of. After 30 hours, a day off my life, some hair loss and 1.5hrs on hold, Front Gate sold me a ticket.