There's a few factors:

- Your gear isn't up to par for the content you're attempting, thus your stats are lower and your cures will be more inefficient.
- Your tank isn't geared well, thus is going to get hit like a truck and will require more heals.
- You simply overcure too much, wasting MP.

Above all else make sure your weapon stays up to date, as that's the item that has the most effect on how much your cures will do. The more your cures will do, the less often you'll have to do it and that'll save your MP.