Maybe it just took them a couple of decades to understand and reverse-engineer the future tech.
Besides, the Empire probably didn't put that much emphasis on science & research since they are constantly entangled in conflicts which binds their ressources. Much like the Klingons in our universe.
It's also the same thing with the Vulcan ship from First Contact
they captured. Although they had direct access to Vulcan technology as early as 2063 (something which humans didn't have in our universe), ship technology didn't seem to be awfully more advanced in the Mirror Universe by 2155. At least the ship design was essentially the same.
So maybe the ISS Enterprise NCC-1701 was slightly more advanced than the USS Enterprise, but the basic design remained the same.