It was, as far as I can tell, the first game which was infinitely procedurally generated yet changeable. Huge procedurally generated games have a long history but in e.g. Elite or Seven Cities of Gold you couldn't modify the world in any meaningful way. The closest is probably dwarf fortress, but there the modifiable world is pretty small (or was when Minecraft came out).

That made it a great game. I think it was inevitable that the first game which combined these two, infinite procedural worlds and free modifiability, would be a huge success. Worth noting also that infiniminer, despite the name, didn't have the infinite part worked out!