If you mean the release of a game that is not a HD re-release, then there were probably a mixture of factors.
First is that many of the games released in the interim were specifically purposed for their platforms. (Days, coded and KH3D were all specifically designed with the DS, mobile phones and the 3DS in mind.)
Second is that the remaining Kingdom Hearts project, Birth by Sleep, was originally envisioned for PS2 and then moved to PSP because of the PSP's multiplayer capabilities.
Third is the development of Final Fantasy Versus XIII (now Final Fantasy XV) talking so much time to develop for PS3. Nomura announced that Kingdom Hearts III would not see the beginning of development until XV was completed, but XV's scale grew until it become unsuitable for the PS3.
Nomura also worked on many other projects during that time.
The combination of these things stretched out the series until the PS4 was already in development by the time Kingdom Hearts III became feasible to produce.