DS9 came out during the 6th season of TNG. From the onset of the show, DS9 used uniforms that looked like this:
However, TNG continued the use of this uniform(taken from episode 10 of season 7):
Why did they not change over? As a military establishment, the change of uniform would have been branch wide. Was there ever any reason, canonical or not, given for this lack of change? At least when they changed the uniforms in DS9 again later on:
Their was reason to not change the uniforms in Voyager, as there was no way to communicate that change.