Quoting this again because
@SJGardner has it right.
It's not like going from the 12th Century to the 21st Century. It's like going from the 5th Century to the 14th Century.
The 20th Century, which we're not too far removed from, wasn't a normal time. Things progressed much faster than was the norm in the rest of Human History. It was an anomaly.
"Necessity is the mother of invention," as the saying goes. Where there's no necessity, there's no invention. So, things stagnated somewhere between
Picard and The Burn. Then there was a century-plus of the Dark Ages. And now they're coming out of it.
Discovery makes a difference because they were able to think outside of the Box. People who lived during The Burn had been stuck in The Box their entire lives. Sometimes you need an outside perspective and someone who doesn't give up. Starfleet of the 32nd Century didn't have an outside perspective until Discovery arrived, and they'd clearly had given up and cocooned themselves. They were in a funk and Discovery snapped them out of it.
EDITED TO ADD: It's not just a
Discovery thing either. Other than being a time ship from the 29th Century, the Relativity on
Voyager didn't seem that different from any other Starfleet ship we'd seen before. The uniforms were different, the layout was a little different, but it was all still recognizably Starfleet. The only difference of significance was the commonality and normalization of time-travel. Once time-travel was banned, that difference was eliminated.