It was easier to learn than the alternatives (UDK, any of the Java engines) and consistently added clearly useful features. Jump to 2019, their networking solu

It was easier to learn than the alternatives (UDK, any of the Java engines) and consistently added clearly useful features. Jump to 2019, their networking solution, UNet is abandoned. Multiplayer is a perhaps a top feature developers/players want. A great open source library called Mirror picks up the slack, but Mirror wasn't made by Unity. As other commenters have mentioned, easily compiling to iOS made it a clear winning choice for game app development.