How about this. Somewhere in the Delta Quadrant a minor not-yet-warp-capable world contains a humanoid species that has been utilizing cybernetics for medical purposes for quite some time, and which has decided to achieve perfect democracy by linking the minds of all individuals together. It actually works, and they produce a sort of utopia. But, as time goes on, children get their implants earlier and earlier, and the collective becomes more monolithic, eventually being dominated by a majority that has never known individuality.
Things become more complicated with the advent of warp drive and the discovery of other species. Peaceful curiosity degenerates into conflict, and this species decides as a group that forcible assimilation of their enemies would be the best way to ensure peace, to make them see the light. It doesn't work out very well. Being connective to the collective against their will only makes the enemy species even more angry, even more violent. The many who voluntarily joined the collective can suppress the few who were forcibly assimilated, but this isn't the slightest bit healthy, and with each forcible assimilation, the collective looses a little bit of its sanity.
This slows their expansion, as the number of minds that are willingly part of the collective must exceed the number forcibly assimilated by several orders of magnitude, or else they risk the collective falling apart. They build their population this way for so long that no Borg alive can remember or even comprehend the concept of individuality, and then they go out to bring peace to the peace of the collective to the galaxy, completely oblivious to what they're taking away from their victims.
Their expansion slows whenever the raging forcibly assimilated minds threaten to overwhelm the always-Borg majority, an occurrence which becomes rarer as their population expands, which would explain why they hadn't conquered the entire Delta Quadrant by now.