star trek - Why aren't we all Borg?

The Borg don't really seem like the type to hold back and in the tiny bit we saw of the delta quadrant in Voyager there were a lot of races technologically inferior compared to the Borg. So the fact that they aren't expanding as fast as they would seem to be able to is a conundrum to me.

Memory Alpha lists the earliest recorded existence of the Borg as being 1484 (although their link doesn't include any details) and at the very least the Borg existed in the year 2063 (First Contact). While it could be said that their methods of assimilation may not have been very efficient in the earlier years, thus providing a potential impedance to their rate of expansion, some time prior to their introduction to the Federation, in the TNG episode Q Who, they had developed a level of technological superiority that any resistance to a major offensive by any known race would have been completely futile.

All of this would seem to indicate that the Borg are for some unknown reason restricting the rate of their expansion. Is there any known reason for this?

Answer

The Borg are choosy eaters.

They are likely several reasons the entire Milky Way Galaxy has not been converted into the Borg Collective. They include:

1. 
   

   The Borg's Transwarp Network while vast and covering thousands of worlds, it is still only a tiny margin of the potential worlds of the Milky Way. Borg space has been considered to be as extensive or a tiny bit larger than all of Federation Space.

   
   


2. 
   

   The Borg are likely searching for races with technological and biological distinctiveness which give the Borg a decided advantage and make that race worthy of absorption into the collective.

   
   
   


3. 
   

   All of those worlds in its space are not likely converted because once a world is converted, all of the developments (the technological one's for certain) are dead due to the loss of individuality. There are no longer any new advances being developed by that species, unless the Borg allow it to retain individuality. (But they would not be Borg, so that isn't likely.)

   
   


4. 
   

   Curiously enough, a race which offers promising technological developments will likely be allowed to continue to develop and merely watched by the Borg if those developments could either be stolen, bought, or reverse-engineered as long as that race was creating new ideas or technologies.

   
   

The Borg were known for plundering interesting tech from worlds on the edge of their space, or from races they could not or chose not to absorb for whatever their reasons. That could be their technological superiority, numerical superiority or because they created technology that made them too useful to bring directly into the fold of the Collective and "Borg-ified".

When a race was deemed near the end of their creative lifespan and had resources which would enhance the Borg's technological or sociological footprint in a sector of the Galaxy, it makes sense to absorb that world and take over that Empire by taking over the worlds that control it. This potentially could ease the transition or destabilize the potential target enough to make it easier to absorb.

If the race were significantly dangerous or powerful, the Borg might avoid them until they could find a means to defeat them as they forced to do with Species 8472 in the Delta Quadrant of the galaxy. Mortal enemies, technologically equal, each struggled unsuccessfully for dominance over the other.

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Summary

Humanity having proven they were capable of developing technology that was at least marginally interesting to the Borg, and in the Borg's opinion potentially able to extinguish itself, was a prime candidate for absorption since, if Humanity in conflict with other races of the Alpha Quadrant were destroyed, the Borg would lose both significant manpower and technological advantages unique to the Alpha Quadrant.

The Borg even went so far as to develop Locutus (the Borg-ified Jean-Luc Picard during their first invasion of the Alpha Quadrant) and other Borg Uniques (the Borg Queens, for example) to treat with Humanity, perhaps to ease the transition of becoming subjected to the Borg's absorptive nanite technology.

To a race that has not developed great technology or has insufficient physical capability, the Borg would be more likely to send a message like this one: "Your species biological and technological distinctiveness isn't enough for us to absorb you. Yet."