Just host one in europe every three years. How is the interest for Junior hockey in Germany? Its a big market, reasonably priced close to Czechia, Slovakia and Switzerland and a very manageable trip for swedes and finns.
Basically non-existant.
To most of Europe, juniors are just lesser variants of the men's tournament, and that applies to all sports. The real World Championships are already more of a side story when you look at sports as a whole, and are mostly interesting for hardcore hockey fans, unless you get the odd cindarella-run by Germany. The fanbase is large enough to host that tournament, but the World Juniors would be hard-pressed to find any sort of attendance.
Germany held the main under-18 tournament in 2022. It managed an average attendance of slightly above 1200 per game (granted, that's not really worse than the other U18 tournaments, but still). Group B didn't see a single game above 1000. And while the arenas were pretty small, they weren't so small that they were full either (around 4500 and 3100 capacity). Landshut saw 12 games, Kaufbeuren 8, only three games in Landshut and one game in Kaufbeuren made it above 50% capacity. You probably could expect a bit more for the U20s, but even filling those arenas would probably be tough even for the most interesting games.
If you are a constant contender for the title, e.g. like Sweden, you can see larger interest, but beyond that you can't expect much.