"Because of the update cadence, and the additional steps you have to go through to be able to release an update on the PlayStation network, we're not going to be able to keep our servers in sync, so you won't be able to just play with the same people,"
Put simply: We're too lazy.
Square Enix has been doing cross platform play for FFXI for 11 years, on 3 different platforms, and soon to be so with FFXIV's reboot which is going on the PC and PS3 and slated for PS4. Even rumours of a Vita version, but nothing confirmed. (I would totally buy that)
There's little reason you can't do this because of a technical issue, unless you're just not smart enough to resolve it, or too lazy to bother.