In fact, Wizards does have the best market research possible for campaign settings - actual sales figures of product lines. 
Honestly, I think that they've made the right choice in supporting the Forgotten Realms (current poll leader) and Eberron.
If they could support another couple of settings, Planescape and Greyhawk would probably be the next couple. However, I have a feeling that Planescape really needs the right design team (who are mostly no longer at Wizards... is anyone from that team, actually?) And Greyhawk has its own problems.
Cheers!

Honestly, I think that they've made the right choice in supporting the Forgotten Realms (current poll leader) and Eberron.
If they could support another couple of settings, Planescape and Greyhawk would probably be the next couple. However, I have a feeling that Planescape really needs the right design team (who are mostly no longer at Wizards... is anyone from that team, actually?) And Greyhawk has its own problems.

Cheers!