Why not?
Because relievers aren't important players? It's like inducting Bunny Laraque into the HHOF because he was the greatest backup goalie in history.
Trevor Hoffman has a career 28 WAR. That's less than Howie Kendrick.
If Chris Sale's career ended today, he'd have thrown more innings with a higher WAR than Hoffman.
A reliever is a part-time player. They aren't important.
That Hoffman got more HOF votes than Mike Mussina (who threw almost 4x the innings and has 82 career to 28) is absolute insanity.
Went back to look at his stats, and then looked at Mariano Rivera.
Insanity @ Rivera. In the post season:
141 IP .70 ERA 21 BB's 110 SO's
Rivera is the only reliever in baseball history that should be in the BHOF.
His postseason resume is astonishing.