Troi was put in the catsuit from day one, to capture the teenage boy market (and most of the rest of the male market). They put her on a uniform at the height of Star Trek popularity.
Seven came in at season 4 of Voyager with the extremely tight catsuit. This is the third series of the 90s era plus a couple of movies, so franchise fatigue started to set in. They needed a way to boost ratings, and they already knew of one surefire way.
With Enterprise, they didn't even try to hide it. T'Pol could have very easily worn a standard Vulcan uniform, which we've seen multiple examples. The catsuit was blatantly used to try to regain some audience. Unfortunately they didn't realize until it was too late that good writing and multi-episode arcs were what we really wanted.