I was never a big fan of the early DS9 uniforms. They just looked too boring and simple-- and even more so on the big screen in Generations (I'm just completely baffled why they chose to incorporate them into that movie). The later black and gray uniforms still had the same utilitarian feel about them, but also looked a LOT sharper at the same time.
Weren't most of the TNG uniforms showing some wear by that point? Not sure what the deal was with the wardrobe budget but there weren't enough if the new ones to go around.
They spent it on brand-new uniforms that were scrapped not long after filming began (they were kind of a cross between the TNG uniforms, the TWOK uniforms, and the future uniforms in "All Good Things..."). http://ottens.co.uk/forgottentrek/wp-content/uploads/2006/06/Robert-Blackman-uniforms.jpg
Does it really need an on screen why? They were smarter looking than the previous uniforms and more comfortable looking than the TNG ones and for me were the perfect TNG era uniform. For the record, because we all like lists my (current) preferred duty uniform list is (top 5); "JJverse" Gold/Red/Blue First Contact TWOK but only as a dress uniform TOS Gold/Red/Blue TMP specifically the short sleeve shirt The All Good Things uniform? Their combadge is indeed the best in my mind also, paired with the First Contact uniform and its even better! And as I have jumped on the lists bandwagon...heres my duty insignia edition "JJverse" TMP Admiral Kirk metal badge (and Kirks STIND version as an extension) All Good Things Generations TWOK officers badge If I hadnt limited it to 5 the TOS and TMP ones would have come in joint 6th. Especially seeming there is a very visible zipper running down the front of their jumpsuits in early seasons and down the back later on! Just as practical as Sisko wearing his 'Captains Vest' under his jacket in Rocks and Shoals, a jacket which is shown to be made of the same material as the jacket and meant to be for alternate wear. Or Kirk having a second Starfleet badge on his vest under his regular jacket in Generations!
For me it would be: 1. Collared TNG uniforms 2. First Contact uniforms 3. JJverse uniforms 4. TWOK uniforms (boy are these starting to look dated) 5. TOS uniforms 6. Early DS9 uniforms 7. Season 1 and 2 TNG uniforms 8. TMP uniforms (the only exception being Kirk's white and gray one)