First of all, OP needs to define what he means by "work". Some random thoughts.
Hockey players carry a lot more muscle, and have problems maintaining their weight through the season. I remember watching a video talking to one of Detroits nutritionists a few years ago and he said they expect the players to lose 10-12 lbs in weight through the season as they simply can't eat enough to replace the calories they expend and retain their muscle mass. I think it was Andreas Lilja they spoke to who explained how he is constantly eating just to try and maintain his body weight.
Soccer players all always on the move. Even at set pieces they'll be walking or jogging into place. And the modern "pressing" style of play means a much higher intensity is required from players than we saw in the past, requiring even higher levels of fitness.
Finally to those who said there's no travel in European soccer leagues....not for the top teams. Sure there's not the ridiculous NHL schedule where you play back to back games, getting into your hotel at 3am, but if you're playing in the Champions League or Europa League, you can be having an away game in the likes Azerbaijan or Russia on a Wed/Thurs, then have to fly back to England for a game on the Sat/Sun.
Squad rotation has been established in soccer for nearly 30 years now and is widely accepted as a thing that top teams need to do. I'm old enough to remember Man Utd seasons in the early 90s where they just ran out of gas at the end of the season and the players were running on fumes, after which Fergie started using squad rotation. Whereas, even in back-to-back games, rotation isn't a thing in hockey. Although that could be as much to do with the Pejorative Slured, head in the sand, backwards, old boys network, this is how we've always done it style of thinking that permeates hockey.