Roll20 has talked to all sorts of companies about licensing their stuff. If I remember correctly most of it came down to the publishers had been burned by doing things with other virtual tabletops(that didn't provide sufficient return on the investment) so they don't want to risk doing things with roll20 until they are sure it will be worth it. I think it would be great to have that stuff available in the roll20 store, but it is really on the IP owners. I think the current suggestion from Roll20 is to send emails to the publisher telling them you want their stuff on Roll20.
