It’s more a matter of convenience.
It looks like Kingdom Hearts HD 1.5+2.5 on the PS4 really will be a pretty basic port. Apparently the collection will just be collating all of the games on to one disc, and calling it a day- for Square Enix, it looks like the draw of this collection is that, between it and 2.8 HD, all Kingdom Hearts games so far will be playable on one system.
“The contents are the same as the two HD remasters in the past. Because six games are recorded on one game disc of PS4, it is easier to play,” director Tetsuya Nomura told Dengeki Online in an interview (translation sourced from PlayStation Lifestyle).
So basically, the exact same games, just playing in 60fps. I guess for a lot of fans, that will be enough? Kingdom Hearts HD 2.8 and HD 1.5+2.5 will both be launching on the PS4 this year. Kingdom Hearts 3 is due out on PS4 and Xbox One at some point in the future.