^^
There probably isn't any incentive for Westjet or AirCanada as a business to allow passengers extended spot overs in St.John's considering that these flights are mostly a refuelling/pick-up-extra-passengers stop to the UK. It would also add to the extra logistics for ticketing and re-boarding passengers etc.
With this new 3days/week London run, this makes two non-seasonal international destinations out of StJohn's now, one in US and one in UK? Cheerio!