I think the tipping point was that 2048 was open-source, on github, and had some low-hanging fruit in terms of improvements or variations. Once the simple changes had been exhausted, the momentum was there to sustain attention for more elaborate but mostly still incremental changes. If everyone had to code it from scratch, we would have seen far fewer versions.

I think it's an interesting study in the possibility space of a design, a breadth-first search by the masses vs. the depth-first search by the Threes developers.