The usual stuff?
I mean, why would the Borg be in any particular hurry with Earth, or with anything else for that matter? They're an ancient lot, and if they really wanted to simply conquer everything, they'd already have had time to do it many times over without undue haste.
For all we know, the Borg had been violating UFP assets every twenty years for the past three centuries already, and they simply hadn't been noticed yet - they're fairly secretive like that, and they don't tend to leave witnesses.
Indeed, the Borg may have been more or less regularly visiting Earth before that, doing their "Yeah, still too primitive" checks in wait of interesting technologies to assimilate. Hence the Grey myths..