Not sure if there's a logical reason. Bruschi is voted in, who shares the career sack record with Thomas (52).
My best guess: They want to stick with "official records" and Sacks didn't become an official record until 2000. Yes, the year 2000. Terrell Suggs has the season sack record (like 24, I think), but Thomas had 27 his Junior Year, for the actual record.
Don't think Alabama has had a coach or player put in the HOF in 2 years. Now, Alabama has had it's fair share put in (because hey, you can't ignore players from a top 5 all time football school), but 2 years is an interesting development, when there are 10-20 inducted every year. Could just be running into players from the 90's and 00's, and Alabama just wasn't very good.