All the other answers are correct, but I'd like to add one more point that no one has mentioned:
Because they don't care.
Picard was surprisingly tricky, and they had some fun hijinks with the Voyager crew, but the Borg have no reason to bring their entire civilization's might toward assimilating Earth, or even the entire Alpha Quadrant. We're just not that important.
The Borg are, above all, attracted to technology that can help them improve. The Federation has basically none. Humans are chock-full of gumption, trickiness, and a can-do attitude, so they often pull off feats that the Borg didn't expect. And because of that, they sent a cube to assimilate Picard and Earth, and another one when the first failed.
But when two cubes in a row were defeated by tricks, and with no notable tech to make the endeavor worthwhile, the Borg probably gave a galactic shrug, made a mental note to get those rascally Humans in a few hundred years, and then turned their attention back to the 10,000 other planets they were in the process of assimilating at the time.