It's also the most lighthearted Trek movie, which I think helps a lot.

And it plays to one of the biggest strengths of Trek: the ensemble cast. A movie has enough breathing room that everyone gets their chance to contribute and everyone gets their chance to shine. Previous Trek movies didn't take advantage of that to the extent that 4 did.

The Motionless Picture, even in its ponderous bulk, doesn't give everyone stuff to do. Oh, it gives the fans long sweeping vistas of the Enterprise that are water in what at that time was a long drought. But if you were a fan of Chekov or Sulu or Uhura, they took a back seat to Decker and Ilia.

Spock Dies at the End is more narrowly focused on Kirk, just because the antagonist is obsessed with him. But even there, our buddy Pavel gets something to do other than "photon torpeeeedos awaaaaye."

Spock Gets Better is... the one that cemented the reputation of the Odd Numbered Trek Movie.