This is because in addition to being the most sold video game of all time, minecraft is effectively open source. It's not literally open source, but you can decompile java bytecode with standard tooling, and symbols are available, either community-reversed, or official.
I think Minecraft's position today has as much to do with the content of the game itself as it has to do with its architecture/Mojang's engineering choices.
The nearly-infinite expandability and modularity of Minecraft is partially due to the simplicity of the art, core game mechanics, and lack of storyline. Good mods are harder to make when the world they must integrate into is more complex.
I think this means a lot of games wouldn't see the kind of sustained success Minecraft has seen, even with more "open" development practices.
I hypothesize that the "open world" craze of years past was an attempt to capture Minecraft magic, but those games lacked the thematic open-endedness that really facilitates Minecraft's longevity.
Click on any mod, and you can see the decompiled source. The mods use function hooking to run before or after certain functions are called rather than the event based system you see in Minecraft plugins.
