It’s relatively common for sports teams to have a rule that they won’t retire anyone’s number unless they’re in the Hall of Fame. The Dallas Stars have no such rule, written or unwritten.
The Stars hung Lehtinen’s number in the rafters at the AAC last fall without him being named to the Hall of Fame, but only one of the other four retired numbers belongs to a player with that honor: Mike Modano’s 9.
Modano was an easy and obvious choice for the Hall, getting in on his first ballot. As for the others, their cases vary.