For several reasons, which I'll cover below.
We have coveralls for exceptionally dirty work that would soil the NWUs, so I'm not sure what you're driving at here. They're even provided at no cost to the sailor.
You don't do manual labor. Some of our men do. Those sailors are afforded coveralls as organizational clothing, even in port. And if they're not, then you can fix that. However, the majority of sailors don't do work in the course of a normal day that you describe above.
Because our public image and public relations actually matters. We like it when towns welcome military bases, not shun them because it brings in greaseballs who walk around in ripped clothing. We like it when people think of the military as a worthy profession from all walks of life, not something you do when you are impoverished or you can't make it in another job. We've actually made a lot of headway as an organization in the last couple decades in this area. Presenting a neat and orderly appearance in public is part of that image.
Aside from that, putting on something you can be proud of raises morale. The NWU Type Is were better than utilities, but missed the mark a bit in this case. I don't hear any of those snickers about Type IIIs. You're deriding uniform changes away from a uniform that looks like a prison uniform as someone who didn't have to wear them. The fact of the matter is that they were overwhelmingly unpopular. Aside from being uncomfortable for most people, our men just want to be able to stop for gas and maybe some groceries on the way home without being screamed at, and they don't want to spend 30 minutes a day pressing their uniforms when they spend 60-80 hour on the ship in-port or 50+ hours a week at sea standing watch. That's not a lot to ask.
You can't have your cake and eat it too. If you want working uniforms to be allowed to be worn in town (which I think we all do), then they have to be presentable for the reasons that I detailed above. And again, if your sailors are painting or working with grease in NWUs, your command is doing it wrong.
Why do you need the MCPON to come do it when you could just do it yourself?
When it came to utilities, the goat locker was mostly enforcing the rules of the time.
I don't know how things work in your community, but in mine, I don't sit on my ass in an air conditioned office and let my sailors get dirty. I can't do the same work they do, but when the aircraft needs to be cleaned and the wash rack is down, I'm out there scrubbing with everyone else. I've spent hours removing floor boards with a screwdriver on det so my maintainers don't feel like all they do is fix the stuff I break. I load buoys on and off the aircraft alongside my AOs with the door open in 100 degree heat. If you haven't done whatever the sub community version of this is, you should probably ask yourself why.
In an aviation squadron, the majority of your personnel are in maintenance; they are doing exactly the types of things that get you dirty or sweaty on a daily basis. They are usually doing it in either extreme heat or extreme cold. My entire squadron deployed to 5th Fleet in 2014 without any organizational clothing because "funding wasn't available". Ever worn dark blue NWUs and black leather steel toes outdoors in Bahrain in July? In 2016 we were in Misawa trying to de-ice aircraft in the middle of a snow storm without winter parkas because big Navy couldn't figure out how to fulfill the supply backlog. Don't tell me that proper clothing items are available and that I can fix it - that's flat out bullshit.
As for your public perception argument - I don't look at civilians wearing stained dickies and work boots as "impoverished greaseballs", so I don't think most civilians would look at service members and think that either. If you do that might be a different issue. And I never said dungarees should come back - I just said looking good shouldn't be a driving factor for the new uniform. Scroll up and you'll see I advocated for the USCG working uniforms because they are everything I mentioned earlier - comfortable, functional, and safe.
BTW you think people were proud of wearing blueberries and they increased morale? Who have you been talking to? Everyone I know hates/d that uniform. Look like shit, fit like shit, ridiculously fucking expensive for what you get, and melted to your skin. Flight suits look cool because of the job they are associated with - objectively they are an ill-fitting fire retardant onesie with velcro patches. Nothing about it screams professional - but people know its military and that's what they respect about it, not the fact that it looks nice.