A better question would be about the Generations uniforms. In some scenes they're in the new DS9/VOY style uniforms and in others they're back in the originals. Did they each only have one new uniform to use?
Out of universe: There was going to be a new uniform design for Generations but at the last minute they were rejected and there wasn't enough time to redesign/make new ones. Now, why they didn't just stick with the TNG ones they already had is beyond me, but...
They decided, I guess, to have a blend of the TNG and the DS9 uniforms but this led to a complication: Cost. As noted above it costs $1000s to make one of these uniforms so only Stewart and Spiner (Picard and Data) got new ones (since, oddly, the movies put those two on pedestals.) Everyone else had to "borrow" uniforms from DS9.
Frakes (Riker) was melted down and poured into Avery Brooks' (Sisko) uniform and Burton (Geordi) was thrown into Meany's (O'Brien).
I think the "effect" they were trying for was that Starfleet was in a transitionary period of standarizing to the duty-uniform seen in DS9... a couple years before just making brand new uniforms all together that were dark and contrasting to their overall ideals.
