As an explanation:
Filters in the Camera app require a bit more computational power, since the effect has to be applied to the live viewing screen. This is 'probably' why older iPads don't include the feature in their Camera app. The same is true for the even more processor intense features like slow-mo, time-lapse, and burst mode.
The Photos app doesn't have this problem, because it doesn't matter if it takes a second or two for the filter to be applied.