Models built for TV in years past often weren't built with much detail, simply because it wouldn't show up on screen anyway. That said, the TOS Enterprise did have a lot more detail than one would expect for a TV show (there are markings and such that are too tiny to see on TV), but it pales when compared to the Enterprise built for "The Motion Picture" which has much, much finer detail.
This touches on something I've mentioned previously- namely, why older TV shows shot and mastered entirely on film still aren't necessarily "HD", even though the medium itself *happens* to be capable of resolving that much detail.
An HD production requires *everything* to have been done to HD standards. If not, it's quite possible that props, makeup et al that were only ever expected to look good on a standard-definition set of the time will show their deficiencies far more obviously under the scrutiny of HD.
There were no doubt good reasons for shooting on film- either technical or aesthetic (film converted to standard-def video for transmission still looks different to natively-shot video)- but decades before HD was even a twinkle in anyone's eye, I doubt they were going to waste their limited TV budget on detailing they (reasonably) assumed no-one was ever going to see.
I suspect that the original Enterprise model was more detailed as it would have been used a lot, and having a higher-quality model in the first place would give them more flexibility in terms of close-ups, etc.
Compare [Star Wars movie props] to some of the ST:TNG props that I've seen that look fine on screen, but when examined closely look like someone gave a 5-year old a couple of shots of vodka and turned them loose with a paintbrush.
Bingo. I bet the one-off single-show models were done as well as required- and no more. (Particularly as ST:TNG was from the shot-on-film-but-mastered-on-video era that- ironically- gave poorer quality than the all-film ST:TOS).