Because it isn't happy enough. Togepi needs happiness to evolve, so its happiness level simply isn't high enough despite its high level.
You can do the following things to increase happiness according to Bulbapedia:
- Walk 128 steps with the Pokemon on your team
- Give it a massage in Cyllage City
- Using Vitamin items
- Level up the Pokemon
- Have it destroy a Soothing Bag in Super Training
- Use Pokemon-Amie a bit
- Use EV-lowering berries
You should avoid letting it faint as that decreases happiness. Using bitter herbs like the Revival Herb decreases happiness in other Generations, though Bulbapedia doesn't list it as one of the things that lowers happiness in X and Y. You should still avoid them though, as Bulbapedia may be just be missing the info.
Once its happiness is high enough, it will evolve into Togetic the next time it levels up.