Of course VOY made it worse by giving us a glimpse of the hundreds of cubes the Borg appeared to have at their disposal. So why send one to attack Earth?
Because it's a very, very big galaxy and Earth is just one very remote part of it as far as the Borg are concerned. Given how vast their territory is, they must be at war with thousands of different civilizations at any given time as they strive to expand their territory.
The Borg at war? The Borg don't appear to go to war. Wars aren't typically decided by one battle or a few battles. The Borg, based on what we know, overwhelms its targets almost immediately. Outside of Species 8472 (or whatever is called) and perhaps the Dominion I can't envision any protracted conflicts involving the Borg against another foe if it decided to use a handful of cubes.
What's more I thought the Borg were smart; I thought they adapted. They lost to the Federation once when sending one cube in the Federation's direction so what did they do the second time they decided to target Earth? They sent one single cube again.
Ridiculous. Of course to be completely fair that decision of theirs only took on the ultimate level of stupidity AFTER Star Trek: VOY went overkill by establishing that the Borg had hundreds of cubes at its disposal. You don't have to be a standout in math to realize that the Borg could probably afford to send a few more cubes in the First Contact film. The Borg sent ten times that much simply to take on Voyager and its crew.
And what does distance have to do with anything? The Borg are all about conquering other races and other societies, adding the conquered's technology to their own collection. By definition the Borg live to seek out new worlds and overtake them. That means lots and lots and lots of travelling. Distance isn't an issue but conquest is a priority...which makes any distance tey have to travel worthwhile.