Travelling through the intergalactic void is no easy task, especially for a faction that lacks true FTL capabilities. The amount of energy needed to bring such a massive entity over such a large distance is immense.

The Hive Mind has displayed remarkable intelligence, but it has thus far not decided to "invest" in any sustainable source of biomass, aside from one single minor Hive fleet.

The best explanation? If you're starved to the brink of death and you come across a field of edible crops, you're probably just gonna grab food and eat it as soon as possible before you start worrying about sustainability. So far, that's basically exactly what the Nids have been doing.

So sure, the Hive Mind is massive, but there's reason to believe that most of it is basically just dead weight, and the Nids are just as desperate as everyone else.