star trek - Why were DS9's uniforms different?

For DS9, they switched to the much less pajamic uniform style.

TNG style: colorful body, collar, black shoulders, no undershirt.

whereas

DS9 style: black body, colored shoulders, grayish undershirt.

Those uniforms were used in Voyager.

Eventually, everyone converts to this:

black body, gray shoulders, colored undershirt

Now, in the first DS9 episode, Sisko, O'Brien, Dax and Bashir are all shown in the TNG style uniform before they settle on DS9 and have chance to change uniforms.

So was the intent that they wear the less formal uniforms on DS9 because -- why -- because it's not a ship? because it's the frontier? because it's not Federation space?

This is confounded since they then go with the new uniforms for Voyager as well as the Enterprise-D in Star Trek: Generations, which seems to be as odds with the DS9/TNG schism.

So what exactly was supposed to be going on in-universe at the time with the uniforms?

Answer

I believe answer has to do with formality versus informality and comfort.

As far as four seasons into DS9, we see the TNG-style duty uniforms still being used. They appear in the episode "Homefront". When Sisko returns to Earth to meet Admiral Leyton, Admiral Leyton wears the TNG-style admiral's uniform and his aide wears the TNG-style duty uniform:

In the background we can see other officers in the TNG-style uniform. In particular, Sisko changes back into a TNG-style uniform for his meeting with Leyton.

We have already ruled out that the DS9-style jumpsuits were intended for use exclusively on space stations, given that they are used on the Enterprise-D in Generations and on Voyager. Since officers based at Starfleet Command are still wearing the older uniforms in the fourth season of DS9, and since Sisko made the point of changing into them for his visit to Starfleet Command, the suggestion is that the TNG-era uniforms are more formal.

In turn, this suggests that officers on starships and Starfleet installations were offered the jumpsuit as an alternative to the TNG-era uniforms, perhaps for comfort while working. Consistent with this, the "Homefront" example shows that the jumpsuits might not be deemed as suitable for official business as the TNG-era uniforms.

So in the years leading up to the fleet-wide shift to the grey-topped First Contact-period uniforms, the TNG and the DS9-style uniforms were in use concurrently, with context dictating which was more appropriate.

This screenshot from the Enterprise bridge in Generations shows both in use at the same time:

Most telling is that Riker begins the film in a TNG-style uniform, but when things begin to heat up, he changes into a DS9 jumpsuit. Comfort is important when you're about the crash the saucer section into a planet!