The Quarians have a very weak immune system, both biologically because their origin planet's has no insect life (I'm not sure how that worked, but that's the lore).
Compounded with several centuries of living on sterile ships made their immune system even weaker, to the point that they require, essentially, self contained biohazard suits.
Putting on and taking off the suits is, in and of itself a dangerous task:
A quarian who wishes to remove their suit must take antibiotics, immuno-boosters, herbal supplements, or the like in order to do so safely, and even then there are inherent risks
Taking it off just so they can be on their planet is, pretty much, a deadly idea for them.
A second reason is: This is a combat mission, they're not just life support suits, they're also combat armor. Why make an expensive combat suit just for one mission?