Here's my take on it;
They've got crappy AI when it comes to combat.
Anyone remember the old days of computer gaming when if you did something really weird in a Real-time strategy the compuet game would freak, and start doing stupid things?
I figure the Borg suffer from the same damn thing. Be it either by the Q sending Picard into Borg space or them blowing up the cube over Earth; the Borg are confused about what exactly happened and why.
So now the Borg instead of mounting a large, concentrated and focused attack, they send one Cube at a time in a straight line towards the enemy HQ. Just like int he old Warcraft games when you'd destroy a starting enemy structure or you would block their unit movements to force them to go around a certain area.
The Borg are simply twits when it comes to long-range space tactics. Anyway what have they had to deal with in their space? The Hirogen, the Kazon, Maalon, Vidiians, Talaxians, and many more which don't seem to either a centralized authority to create a large space fleet or have a means to operate in a group. Only species 8472 provided the Borg with a challenge, and they still used the basic tactic that Gheorghi Joukov used in WW2, hordes of men.
At least, that's my reasoning.
"The misery of being exploited by capitalists is nothing compared to the misery of not being exploited at all."
-Joan Robinson, economist