I think that it's because vi (and its ilk) already occupies the ecological niche of modal editors.
The number of people who prefer modal and haven't yet been attracted to vi is probably 0, so the hypothetical vi competitor would have to be so great as to make a significant number of vi users switch. This isn't likely. The cost of switching editors is huge and the vi-s are probably already as good as modal editors go. Well, maybe a significant breakthrough could improve upon them, but I find this unlikely.
