What is more military? If you filmed TMP and TWOK with TOS uniforms what would be different? The characters don't act differently. The Enterprise gets a weapons and shields upgrade in TMP. The military lingo and command structure is the same.
Nick Meyer wanted it to look more naval, and Gene Roddenberry was furious enough with what they did with the TOS Movies that he went to his lawyer to have 15 minutes cut out of TUC. Which ended up going nowhere since he died two days after seeing an advanced screening.
I read a book by Yvette Fern,
Gene Roddenberry: The Last Conversation, where he went so far as to say TWOK wasn't Star Trek.
It's probably more about optics. And the fact that Gene Roddenberry came to believe The Future would be non-violent. Then you have all the movies except TVH having traditional Bad Guys who need to be stopped. And eventually the TNG Era followed the same path.
He wasn't winning this battle. <-- Irony
There's probably also the soundtrack. James Horner's music in TWOK (and TSFS) sounds like "The Navy In Space!!!" and his style is very much probably not what Gene Roddenberry wanted. James Horner also went on to score
Aliens, the most militaristic film of
that series, complete with the Space Marines. And I suspect he
really hated when TUC had such military hardliners in Starfleet who didn't want peace and conspired to keep it from happening.
EDITED TO ADD: The Space Program had past is peak from the Moon Landings, while Reagan was expanding the Hell out of the military. It's probably not a radical leap to conclude that Gene Roddenberry began to hate the idea that the only way Humanity could expand into space would be if the military expanded out to there.
In his own personal head space, Gene Roddenberry drifted pretty far from what he originally created. Probably another reason why when he created TNG, he wanted people not to look at TOS.