But that doesn't explain why they don't send more than one cube at a time to Earth.
I blame Voyager writers and bad writing for introducing the idea that the Borg has "thousands" of cubes. Before that show, I just always assumed the Borg might have had 3-4 cubes in the entire galaxy and couldn't send anyone at Earth after BOBW which had been a crippling blow.
Borg space is huge. They didn't have transwarp hubs in TNG (which is like hyper-accelerated transwarp) so it took a while to get to Earth. Also consider while they may have thousands of cubes, they need most of those to maintain their space, probably assimilating drones and mining worlds for building materials. Also consider ther simply may have been many closer/better targets to deal with (like Arturis' race) or dealing with the rabble trying to topple them. Maybe part of the Borg empire is bogged down in a war with another race, one not so strong as to decimate them like Species 8472 did, but strong enough to resist and hold the line, but with technology they really want. When they really want something, they send more than 1 cube. The BOBW cube might've been a test run to assess how easy it would be to assimilate the Federation. One would think there would've been a parallel BOBW for the Klingons (test run on the Klingons), yet we never heard of it.
I think so much of the problem comes with the mistaken impression that these were like machine gods, nigh invincible, instead of being an advanced yet in some ways simple race that did have a huge area of space and which were likened to a force of nature at times. They have their weaknesses which other races try to exploit (technological with Species 6xxx whatever and biological with the Brunali). Voyager had a former drone and the desire to survive against the Borg. They also had some Borg tech onboard (Astrometrics) and a chance to study Borg tech close up on many times (Unity, Scorpion leftovers, scanning One the Drone, etc). They had the will to exploit every advantage against the Borg, even trying to stoke a Borg civil war via Unimatrix Zero (that would've made for a better "Endgame" than Admiral Janeway, who drank so much "coffee. black" it altered her personality to be just like it).