>>713756489Tali mentions it. They're symbiotic with their home planet's flora. Colonising planets would make them sick but they'd adapt, but when they fled the quarians spent so long in sterile space that it ruined their immune system. It would take generations to adapt to a new environment that isn't their original one.
So the quarians are stupid for messing up their health and even more stupid because they keep thinking their generation will be the one to retake the homeworld.