Canada just doesn't have any good doubles players other than Nestor. I think they probably figured that it wouldn't matter who they put with Nestor, that they couldn't get a good enough pairing for a medal anyway.
hope that's not it. AFAIK the COC's primary goal is to own the podium. Nothing against Pospisil, but this isn't a feel-good charity. Nestor should be able to name his partner and he should choose his best hope for a medal.