Ramming seems like an obvious thing to try when all else is failing and ships around you are being destroyed and your own future doesn't look too bright, either. Yet the Cube survived Wolf 359. So it's only natural to assume that the Borg know how to defeat ramming attempts.
Remember that the tractor beam seems to be a favored tool of the Borg. It sounds rather likely that they could grab any approaching ship in such a beam, be the approach at low impulse or high warp, and thus render her harmless.
Why couldn't they have transported an Explosive/Torpedo over there ?
Because that would have hurt the Borg, so they would have stopped it? Beaming people there and back did not hurt - it only resulted in the loss of a few perfectly expendable Drones and the gaining of new data on Starfleet phaser settings and tactics.
Timo Saloniemi