People went to see the first Star Trek movie because they liked TOS, and then what we got was ... nothing like TOS. Kirk isn't at the top of his game, Spock is off trying to starve his emotions into submission, and the characters spend WAY too much time staring at the viewscreen in awe and way too little time interacting with one another or trying to solve the problem. It felt to me as if the special effects were the star of that movie, and the human stars were extras in their own film.
I think it's fascinating the way the creators of some of the most beloved science fiction series seem NOT to understand what they themselves have made or why people liked it. Roddenberry gave us a Star Trek movie that many fans disliked, and George Lucas gave us a second Star Wars trilogy that's almost universally regarded as much weaker than the first one. How could they create greatness and then not quite understand what they, themselves have made? Just one more way in which people are really interesting.
It isn't so much that Roddenberry "didn't understand what he had created" back in the 1960s. It's that he didn't want to simply rehash it. He wanted to take it in a new direction. A conscious choice.
In terms of a difference in tone, I do think the script's origins as a TV pilot are at least partially to 'blame' for it being different to TOS. A lot of time and money had already been sunk into developing the Phase II concept, to 'adapting' the Star Trek format to what audiences in the late 1970s might expect it to be, taking into account how television had evolved in the ten years since. So, when the pilot was green-lit as a theatrical movie instead, they were effectively being told to shift gears... but Roddenberry, understandably, wanted to try and retain something of what he'd been working towards those couple years working on developing his concept to it's next stage. He might still have had an eye on maybe reviving the Phase II concept too, and was simply working towards that goal rather than trying to recapture something he'd already done ten years before.
I say that not as a criticism. I say it as an observation. Gene's parameters of what he wanted Star Trek to be had changed, and he was entitled to explore where those changes took him and his concept.
George Lucas must have felt the same way with Star Wars. As he developed his prequel trilogy further, he evidently was taken by certain ideas that wouldn't have occured to his younger self, but which intrigued him at that point in his life, and he wanted to see where those ideas would take him within the boundries of his writing. Some people may not like
the end result of Lucas' reimagining, they might not like
what Gene Roddenberry did with TMP. But they are both 100% what the creators intended them to be. The fact that the audience may not have been completely on board with that development is neither here nor there.