There were plenty of alternatives: C4, Torque, Leadwerks, ShiVa, NeoAxis, Esenthel, Panda3D, Cafu, Crystal Space, and others. Unity gained popularity because it

There were plenty of alternatives: C4, Torque, Leadwerks, ShiVa, NeoAxis, Esenthel, Panda3D, Cafu, Crystal Space, and others. Unity gained popularity because it supported mobile devices at a time when the smart phone market was only a few years old and mobile games were profitable.