Either way it still requires far less developmental resources to make a HD re-master and that's even assuming that the same team making this port is the same team responsible for KH3. Even if it is the same team it's likely only a very small portion of that team that's working on 2.5. Considering 2.5 ReMIX was hinted at in the special ending of the Japanese 1.5 ReMIX, they were very likely working on this as work was wrapping up on 1.5 - hence it's relatively quick turn around considering 1.5 only came out on Western shores last month.

Besides SE have already come out and said that there are developmental difficulties considering Nomura is involved in both XV and KH3 so in all honesty it's very likely we'll see XV way before KH3. Sure the waiting time for KH3 has been immense but a small team working on 2.5 is hardly going to shave years off of the development of KH3 if they were moved over to KH3 instead. Judging by Nomura's interviews at E3 they were still hammering out the basics of the engine and tools to make KH3 so some of the team is likely just working on 2.5 due to it's relatively easy turnaround and once that's done, and the base engine and tools are available, they will transition onto KH3 once development is in full swing.