I don't know, and honestly, I don't watch enough of either(though I have been unimpressed with both players in the playoffs). All teams make questionable picks for their national roster (bias, "nepotism", and attitude all factor in).
One possible narrative is...Bouchard or Harley would not be Team Canada's big weapon on the blueline; he'd be getting secondary or even tertiary minutes. Harley has a proven track record in that space; he can play in all situations and already plays 2nd fiddle to Miro Hiskanen. How would Bouchard do that role...maybe without any PP time?
Another thought..perhaps Bouchard is one of those players with great numbers but is hurt by context. Rightly or wrongly, he's seen as a product of McDavid and (to a lesser extent) Draisaitl. Even Tyson Barrie put up great numbers in that position! Is Bouchard even his team's best defenseman, or is that Mattias Ekholm, who can play in all situations? It's telling that Bouchard is not considered an untouchable by Edmonton fans (though partly that is because he's up for an extension).