There wasn't a real answer, the writers needed an excuse for the Borg to not just blow up the ship in a nanosecond and decided it was the Queen's obsession with Seven.
TNG did the same thing: there were numerous points in BOBW that the Cube could've destroyed the Enterprise but never did.
But a Sovereign would have lasted better, it destroyed that Borg Sphere in FC like it was nothing.
I thought they let themselves be destroyed. After all the damage done during the running battle with Starfleet, the Sphere must have been less than 100%.