This my head canon… In the movies they were working on transwarp drive on the Excelsior. The warp factor on transwarp drives is different from a regular TOS warp drive.

When Sulu put the hammer down on the Excelsior in ST VI, it proved that transwarp was just way better than regular warp drive. For whatever reason they couldn’t make transwarp work on the Constitution class so most of them were decommissioned.

By the time of TNG, pretty much everything is transwarp, so no one bothers calling it that ,they just call it warp drive since it became the standard. The transwarp factor is used everywhere, but everyone just calls it warp factor.

So it’s kinda like a change from imperial to metric units.