Because the vast majority of its customer base are Final Fantasy fans.
Thats quite a generalization, but its not exactly false.
Rereleases make perfect sense on the successor of the previous platform. There is no established market for these games on any of the current gen systems, and the cost of advertising would be a considerable detriment. With regards to that, expecting Nintendo to advertise a mutliplat is not realistic, Microsoft might but that is unlikely too. The only reason KH3 is even on XB1 is cause SE mismanging funds and trying to keep from being capcom.
It would lose money on any other platform, so it makes perfect sense as an exclusive. The issue is that unless SE ports 1.5 and 2.5 to the ps4, then it probably won't be playable because Disney has a contract that prevents KH from being sold digitally.
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