It's possible that the WoK type uniforms were phased out and then brought back again
a few decades later due to popular demand or whatever. We've just never seen any of the ones inbetween.
It's also possible that the WoK style uniforms were only kept in use for certain ships or situations, with different uniforms (or variations) used for starbases, outposts, long missions, etc. (as per DS9's jumpsuit style), which we also never saw.
On the other hand, maybe everybody really did just stick with the same uniform for 70 years. *Shrug.*
I want to see the faces of Picard and others, the day they show up for duty and have a memo from Starfleet quartermaster in their Mailbox.
"Oh, finally new uniforms, I started to feel like my grandpa in those old threads. Computer, replicate new uniform pattern 2354 Alpha! They took 80years to come up with a new design, I bet they are awesome!"
Replicator materializes new spandex uniform.