They publish most major Role-playing games, along with some trading card games.
They publish D&D, which even if it is the largest game, doesn't make it a majority of the games. From their site they don't even publish any others.
Shadowrun, Paranoia, Pathfinder, L5R, Traveller, Eclipse Phase, Dark Heresy/Deathwatch/Rogue Trader, Exalted and oWoD AND nWoD are all produced and published by other companies, so I don't know how D&D alone constitutes "most games"
Anyway, I haven't had a campaign in ages. It's very sad, I never got to play my doctor-rogue in pathfinder.