I think a lot of it comes down to football being one of very few places in Scandianavian society where it is acceptable to show emotions etc, so it kind of just explodes with suppressed emotion. Another reason is that every club (in Sweden, but most in Norway are as well) is membership owned which does a lot for fan engagement. Hundreds of fans meet nearly every week to produce flags, prepare tifos and so on because it is truly their club, so if they want it to be lively and intimidating, they know that they have to do the job themselves.I can see why ultras in some countries go f***ing nuts for football due to their stereotypical temperament etc, but I fail to understand how Scandinavian clubs also have insane ultras. Such mild mannered, humble folk stereotypically
Ticket prices also plays a part I think. Anyone can go to football in Allsvenskan. Apart from the biggest games, it is always possible to get a ticket for under £10. Any club board that tries to hike up the prices has a very real risk of getting voted out by the members of the club.