I am not sure if it is any different other years. 42% are paying attention is actually quite a large amount of people.
A couple of factors. The economy is downright abysmal in most places in Canada. With people having difficulty paying their bills and paying for groceries this might be a bad time for people to watch McDavid possibly win his first Cup since there are other issues on their mind.
Secondly, I have no idea why the NHL does this every year but their postseason goes too long. It all starts with the beginning of the season. The first game of this NHL season was on October 10th. Why? Why not start the NHL season on October 1st? It coincides with MLB ending their regular season and it gets you the jump on the NBA. The NHL playoffs didn't start until April 20th. And there is no reason to wait 6 days to start the Cup final. Start it three days later, nothing more. It isn't the Super Bowl. The weather is and can be sweltering in parts of Canada at this point, let alone warmer parts of the U.S. There is a reason viewership is lower in September in the NFL. The weather is still nice, people hit up the final time they can go to the beach. Ditto in Canada, this is beach weather now, campfire weather, camping weather, etc. The Cup final should be done even if it hits Game 7 in early June.
Also, look back at how things were done in the past. 1991 the Cup final went 6 games and was done May 25th. 1992 it was a sweep and people lost their minds because it was June 1st (most notably because of the 10-day player strike near the end of the season). 1993 the playoffs started on the 18th of April as well and ended June 9th. Why did it end sooner than what would be Game 5 of the Cup final this year on June 18th? Because they started the Cup final three days after the LA/Toronto series ended. They also had all of the games two days apart, even with the long travel of L.A. to Montreal. Why do all of this three days in between games stuff? It isn't needed.