That's a lot of money that SE would have to dump to get such a crossover, and have to pay royalties every month/year to maintain permission for the costumes to be in the game. It would drain a lot of money from the development budget.

It is easy for SE to do crossovers within their own game franchises because they do not have to pay someone else.

Besides, if I wanted a crossover I would prefer one with the Hyperdimension Neptunia series. It would be A) Cheaper and B) would add yet another company character to the Neptunia lineup. Not to mention all the costume and weapon glamours we could get from that... Neptune's/Noire's/Nepgear's/Plutia's CPU sword for DRKs, Neptune's/Noire's/Nepgear's/Falcom's human form swords for PLD, Vert's spears for DRG, Blanc's hammers/axes for WAR, IF's Qatars for NIN, Peashy's CPU form gauntlets MNK, ROM's/RAM's Staves for WHM/BLM, Uni's rifle collection for MCH... actually I think only AST would be out of luck when it comes to a Neptunia Glamour.

Costume glamours you got everything ranging from the CPU human form outfits, Maker outfits, and CPU outfits.

Would not be hard for such a crossover event to happen, I mean it would not be the first time a dimensional rift has opened in the Neptunia series.