Biggest reason is that there weren't any decent competitors around as Unity built momentum. At the time it was Unity, a (difficult-to-work-with, difficult-to-ga

Biggest reason is that there weren't any decent competitors around as Unity built momentum. At the time it was Unity, a (difficult-to-work-with, difficult-to-gain-access-to, expensive) proprietary engine, or something vastly less feature-filled like Game Maker.