Because it would adversely affect gameplay and scenario framing.
Episodic games or games that are chapter based like XIII have hard limits in place in order for the game experience to be optimized for the first run and not power creeped beyond reason. If free reign were potentially given for abilities, materia and stats, it's conceivable a player could make their characters strong enough to the point that they have end-game level stats by the time they just enter part 2.
So either the developers game design off that possibility and make enemies ridiculously difficult to surpass the limits of the previous chapter, accept that they may craft a lukewarm game experience due to the nature of episodic releases, or they can only allow part of the experience points to carry over and reset materia, or they just cap the AP and Experience and prevent the problem from the start.
Either way, something will be lost or limited.
Capping the experience is the cleanest resolution because no one is gonna want to see their hard work and experience removed and most players aren't looking to power level and then face a game where if Cloud sneezes on an enemy, it dies. Developers likewise can only arms race the battle mechanics so far before it becomes unwieldy and absurd. 99,999,999 damage being dealt by Thundaga XVI off a level 348 magic materia is gonna just look silly. Data overflow may be an issue.
The episodic release is already wildly different and at odds with how the original was done so if they are gonna need to change and accommodate the experience and AP systems to match it, they might as well do it now from the start. They need to make the game as enjoyable and balanced as it can be.