They probably never fully developed the technology at the time or they were never able to (I'm not that well read on old school ST lore, I'm pretty knowledgeable about everything past TNG and Enterprise era). I know one of the biggest things that came out of Voyager was them stealing Transwarp technology from the Borg to try and make their trip home a lot quicker (if I remember correctly they did but never fully integrated it into their warp drive, they shaved a couple of years off their travel time before it gave out if I remember correctly). All I know is that tech has seen a significant advance in STO (shit we have Quantum Slipstream drives now (which enables the Transwarp ability on the space map, normal transwarp is instantaneous between transwarp gates)), mostly due to the Iconian threat and the intervention of time travelers (yea, the Temporal Time War is mentioned and we do get to fight the Xindi as well. Hell my Science Officer is flying a ship from the future and she's from Kirk's era (yea I know, bonkers)). I just find it funny that our ships are more powerful then ever before (yea this is a game so I understand that aspect) yet warp travel is still stuck at 9.97 (though I do remember them explaining why this is in Voyager I just don't remember the specifics, this is the reason they turned to transwarp).