Two diplomatic incidents in multiple centuries really shows how half hearted quarian settlement attempts are... Unless ME writers weren't using Uber's estimate and infact habitable candidates for the quarians were very few and far between because they existed as a genre conceit.
Mass Effect has (sadly) often been more of a tell then show game in a lot of ways. Like we are
told that there are
trillions of lives which implies thousands of planets but we are never really shown the signs of trillions of people and thousands of planets and shown many things (like the Earth Alliance mattering in the slightest with a population in the low billions) that contradict it. Then we are told stuff like less then 1% of the galaxy being explored which
sounds tiny until you realize there are 400
billion stars in the Milky Way so 1% is still
four billion stars which even if one in a million is habitable also implies thousands of habitable worlds. So there are multiple sources
saying one thing and multiple sources
showing another and there isn't really a clear answer either way.
In this case however while the feasibility of the idea is kinda stupid what we are told and what we are shown does match up in the case of the Quarians. They are space gypsies hated and discriminated against by everyone and forced to stay perpetually on the move because everyone hates them. The only known example of the Quarians trying to colonize a planet results in their rapid expulsion under threat of military force. We know that in
at least one case the Volus tried to get the Citadel Council to force the Quarians to hurry up and leave one of their systems and only failed due to them needing to recover from the Battle of the Citadel.
If I was to try and explain things I'd say that the Quarians in the early days wanted to reclaim their homeland and weren't interested in settling down. Then as the generations who were born on starships started replacing those born on planets they started searching for a new homeland, having recognized the impossibility of dislodging the Geth by now. Except Quarians already are very limited on where they can colonize (dextro-protins and weak immune systems) and the century or so of life on clean starships has made that even worse (crippled immune systems). This compounds with having limited resources, their ships are a century old at this point so much of the fleets resources are already going into maintenance, so their ability to search for new planets is highly limited, ships venturing off alone risk destruction and the fleet needs to stay in civilized space for ready access to supplies. So the Quarians are restricted to worlds discovered by other races but not yet claimed which limits their viable choices to just a handful even across centuries of searching; a small enough number that it was practical for the discriminatory elements of the Citadel races to disrupt any attempt at colonization.