"Federation scientists speculate that the Borg's use of temporal technology is a fairly recent development, since the Borg have never used it before. Most likely, the Borg developed the technology after other means of attacking the Federation proved unsuccessful. They adapted to the new circumstances and created a new means of attacking the Federation through time. It is also possible that the Borg acquired temporal technology by assimilating a race that possessed it, possibly the Krenim, although the Federation is not yet aware of the existence of the Kenim Imperium.
"As to why the Borg have not used their temporal technology more often, or made a more concerted attack on the Federation, the most popular theory is that the Borg are cautious in the wake of their last defeat. The attempted intervention in 2063 resulted in the death of the Borg Queen, the coordinating force of the Collective. This was a serious blow to the Borg and may have made them wary of future attempts to tamper with time. It is also likely that the Borg have many of the same difficulties in predicting the outcome of certain temporal changes as the Federation, so they are unwilling to attempt some interventions because of the possible repercussions.
"...One interesting new limit on the Borg's time-travel abilities for Narrators [GMs] to keep in mind is their new 'dependance' on humanity. In 2374, the crew of the U.S.S. Voyager provided the Borg with the key to winning their war against Species 8472. If the Borg assimilate humanity before this key event, and do not insure that the information is passed on to the new timeline, it is likely that Species 8472 will destroy the Borg Collective and much of the galaxy! Therefore, the Borg must proceed carefully when it comes to altering history, at least as much as anyone else does. - "All Our Yesterdays: The Time Travel Sourcebook, p. 79