I tried to support a previous answer and could not (so submitting an answer).
The Borg did not jump back in time earlier, because they didn't expect to lose.
Have they ever expected to lose? No, they expect to overcome every single time, and assimilate to continually improve a very successful plan (do you understand what the phrase 'Resistance is futile' means? When would you use it... when you have a chance, a good chance, or when your offense/defense/technology is vastly superior?
If you only have a good chance, you don't waste time talking to the enemy you go for the win!
They didn't do it proactively, it was a reaction (Dang it Locutus, that's going to destroy the cube!! What are our options, destruction is imminent -- the queen can escape in the sphere? (that doesn't do much for her, if starfleet can just shoot the sphere). But it can go back in time, and they can't follow. Good solution, otherwise, she's dead -- 'make it so' (yeah, they copied 'Locutus', he was assimilated -- so that's allowed)
I'm not an expert on the 'Borg' -- but they have always seemed to pick fights they can win, to gain more technology and combine it with what they had. They were initially presented as a problem the Federation was inferior too -- so Q thought they were superior. Can you picture the Borg coming in, and instead of the traditional statement they said "We are Borg -- may the best species win! We've got a new strategy, we might surprise you and win! We're gonna try!" The attitude, and approach are totally different.
Time travel is probably a complicated risk (anything that's a risk is a lower priority to the logical hive-mind, stick with the proven plan... destroy and assimilate)