People don't go 10 man because there's no reason to make it harder on yourself for the same rewards. The difficulty difference is obviously much greater, but how many people do you think would run heroic if it dropped same rewards as normal? When the majority of players roll a spec that they enjoy less just because it does a few percent more damage, do you really expect people to intentionally make it more difficult for themselves just to have a bit more of personal impact on the encounter?
The difference between 10 and 14 man is not large enough for people to make it harder on themselves, it's still roughly the same size, just a bit worse. Back when you only had 10 and 25 man options, during the period of 10 mans being harder you still had tons of people doing it, including myself, because the difference between 10 and 25 was huge.
If the difficulty level was the same, you'd see a lot people running it, even if just because of easier logistics.