The Mass Effect series was supposed to have more content focused on the Rachni race, but plans went awry in order to continue the story of the Geth. Spoilers for the series within.
Drew Karpyshyn, one of the original Mass Effect project leaders and key writer of the first game, told Kotaku that the Rachni had to take a back seat.
“The Rachni choice is something we really wanted to have massive repercussions down the road. Unfortunately, the realities of working in the industry – schedules and budgets and deadlines – mean that we can’t do everything we want to do,” he said.
“I will say one of the ways we tried to offset that was with the inclusion of the Geths and the Geth party member that came in Mass Effect 2, was Legion.
“He was never originally planned and the Geth were going to be sort of pushed to the side. But people really responded to them. We wanted to explore the Geths and that was one of the tradeoffs of not doing more with the Rachni.”
In the first Mass Effect, players have the choice of killing off the last Rachni or accepting her promise of peace and let her go freely into space, presumably to proliferate her species.
Apart from a brief mention in Mass Effect 3 of friendly Rachni troops in the final battle against the reapers, and in the occasional bit of media chatter, this choice has little effect on the RPG series’s conclusion; the Rachni turn up regardless, among the Reaper forces.