If you're just going to be a DH, you better be among the offensive elite, year in and year out, in your time. He was very good for a long time, but rarely elite.
Check out these two pitchers. One is in the HOF, the other isn't.
Pitcher A: 270-153 W-L, 3.68 ERA, 2813 SO, 7.1 SO/9 innings, 3.58 SO/W, 8.7 H/9, Postseason - 7-8, 3.42 ERA
Pitcher B: 216-146 W-L, 3.46 ERA, 3116 SO, 8.6 SO/9 innings, 4.38 SO/W, 8.3 H/9, Postseason - 11-2, 2.23 ERA
Pitcher A is a HOFer (Mussina). Pitcher B is Curt Schilling. No freakin' way.