It's always irked me that A-35 and I-89 still don't connect; A-15 and I-87 currently connect to each other, as do A-55 and I-91, so why the long wait?
Limited money, limited need, and limited political clout from the town of Pike River (if they're even bothered by traffic through town).
Would it be nice if the autoroute extended to the border? Yes...but the existing roadway seems to adequately meet the need, at least based on the traffic volumes I've seen when commuting.
I can think of other transportation projects to spend money on, like getting the customs preclearance centre up and running in Gare Centrale and the Montrealer train service restored. (The biggest problem with my work trips up to Montréal is finding a hotel near the office, within the corporate expense limit, that has parking.)
(I also wish that they'd have the southbound NEXUS lane open for longer hours at Highgate Springs, but somehow I don't think funding from that would come from either the Canadian federal or Québec national governments.)