Supposedly Rick Berman et. al. didn't like the way the uniforms worn on Next Gen from seasons 3-7 looked on the big screen in Generations... in fact, they had designed new uniforms for that movie but didn't use them. So, when they got to First Contact, they introduced the gray-tops as a suitable (no pun intended) replacement. DS9 got them at about the same time because as Khan said, "We're all one big, happy fleet."