it comes down to philosophy of voting, and the way voters look at the HOF.
should the Hall be reserved for the very Best of the Best - or should it include players who were very good, but maybe not The Best? which ties into another debate - what about players who were great, but had shorter careers, versus players who played a long time and accumulated more career stats.
after looking at some of the other pitchers in the Hall, Sabathia belongs. over 3,000 K and 250 Wins puts him above a lot of other HOF pitchers - albeit some with much shorter careers.
with relievers, it's different because of changes in the game. I think Wagner is only the 9th relief pitcher to be elected. But if you look at Billy Wagner's stats, they're comparable to other relief pitchers who are in the HOF. 8th in career saves with 422.
FWIW - Joe Nathan is currently 10th on the all-time Saves list with 377. he had more career saves than Rollie Fingers and Goose Gossage - who are in the HOF.