It's also possible that TMP suffered retroactively by comparison to WRATH OF KHAN. When TMP debuted, there were no other Trek films to compare it to, so sympathetic reviewers compared it to 2001.
(I also believe that there's a fair amount of CLOSE ENCOUNTERS OF THIRD KIND in the movie's DNA, but that tends to be overlooked as people forget what a big hit CLOSE ENCOUNTERS was at the time.)
(I may have said this before, but...)
The late 70s really saw a big change in the way movies were made, chiefly in terms of pace. TMP is extremely slow in comparison with TWOK, but also in comparison with other fantasy adventures of the time, such as Raiders of the Lost Ark, Aliens and the Star Wars movies. I see it as really the last gasp of the big screen epics of the 50s and 60s: the Bible movies, the David Lean films, etc.
Close Encounters has been mentioned a lot as a point of comparison, but that film differs from TMP in some important ways:
1. Even though it's not action-packed, the directing style is much slicker than Wise's, full of little moments of crisis and suspense, and the staging is much less static.
2. The big light show at the end is an emotional pay-off to the journey of the film, whereas TMP's lacks dramatic content (it's ten minutes of one emotion), and is only a stop-off midway to the end (I'd compare it with the journey upriver in Heart of Darkness in that way).
3. Although we can talk about the emotional experiences of the characters in TMP, these things are really observed at arms length, brusque dialog in sterile corridors, whereas TWOK makes a point of getting us emotionally close to the characters from the beginning: Chekov's unexpected discovery of an old enemy, followed by helplessness and torture. Kirk traumatised and lost on his birthday, with his friends gathered round to comfort him. Scott's nephew joining him in the engine room (though they really needed a different actor).
TWOK takes place within an emotional, social world, whereas TMP takes place in a vacuum. This is part of the reason it's compared to Kubrick's 2001, but that movie was designed to be a philosophical, mystical experience in the vein of Olaf Stapledon, whereas TMP is at heart a Space Patrol adventure, which requires easier identification with the heroes than I think TMP allows us.
I'd say part of the problem is that TMP is a TV episode writ large, and in TV series of that time there was no expectation of character development. The heroes would face a problem, solve it, and finish up basically the way they started. It was TWOK which established a precedent for Trek movies being based in the emotional journey of the characters.