They must have been still examining it and the other technology from the Borg and the future. They didn't make ships with quantum slipstream drive in the novels until 2381.There are a variety of reasons why the Enterprise-E might not have the technology. One possible reason is that there could be treaty stipulations preventing the further development and employment of the technology (such as the case with cloaking technology).
Another possible explanation is that the technology is being kept secret and in reserve to be use specifically against future Borg attacks. As long as they can keep the Borg from assimilating the tech (or anyone with a high degree of knowledge about the tech) they maintain an advantage.No, it was confiscated by the Temporal Investigations department, to prevent a mess-up in time. Don't want to create a universe-destroying paradox again, you know. The Federation would need to invent it themselves by 2404, unless it is a predestination paradox.
188.8.131.52 23:25, July 9, 2011 (UTC)
As i understand it the Enterprise-E (the Souvereign class, so not even the first of the class to exist) was developed at the same time, paralel, as the USS Defiant was, and with the same purpose. So when Janeway got back after blowing up the Unamatrix the Enterprise-E or at least it's class (Souvereign) allready existed. If anything the tech Janeway brought back can be retrofitted to the Enterprise-E and or the Defiant. It does explain why there are quantum torpedos but no quantum slipdrive.
The defiant had Ablative Armor and Quantum Torpedos.
To add to the above:
It is also possible that some version of the "temporal prime directive" already exists in Starfleet, and as such there are rules against using such technology (in addition to the possibility of creating a paradox, there might be some kind of attempt to keep the timeline "clean"), if it is not simply destroyed outright. There is also the possibility that, even if they can build it, they might not have the proper infrastructure to be able to use it right. An admiral would have a LOT of explaining to do if they were the one to authorize the deployment of such technology on the flagship of the Federation (which is also one of the few members of the most expensive and powerful ship class they have ever created) and it turns out that it causes the warp core to explode due to some particle interaction that they didn't expect causing containment to fail.