Unfortunately, the decline in playoff viewership isn't a new thing. It continues the general decline the NHL had throughout the regular season. Interestingly, the Four Nations tournament was a massive success among viewers, according to a related article. Hockey can obviously be a viable product; it seems to depend on how it's marketed and who's playing whom.
People have pointed out the absence of legacy American teams in the playoffs as a possible reason for the drop-off, but that doesn't explain why the regular season suffered from a similar drop.
Could be because those same legacy teams were bad in the regular season as well.