I've been part of a community of hobbyist/indy gamedev since 2002. At that that time, there were very few engines usable by hobbyists due to price constraints: - Quake 3 Engine for FPS-like. Other FPS engine were moddable, but as the source code was not available it limited the possibilities. - RPG Maker for old school JRPG - Adventure Game Studio and GameMaker for Sierra/LucasArts adventure games. I could probably include ScummVM. - Mugen (and another one I forgot the name) for VS fighting games. I certainly forget some others.

These engines being tailored-made for a specific type of game, people wanting to do something else had to create their own engine, often in C++, sometimes in Python or Lua, reinventing the wheel most of the time. The barrier for entry being so high, game designers had to team with coders, but their end-goal not being always aligned (creating the game vs learning how game engines are coded) this lead to the demise of many projects.

With Unity, a general purpose game engine became available for a very low price (in particular compared to Flash). Many game designers saw it as a way to work on prototype without needing to recruit a team of coders. And for many coders, it was a way to try their hand at game design, while being able to push the engine further using C#.