It used to be on there, and if I remember correctly the reasoning was that it continued a meta of AP tanks that wouldn't fight you, they'd just slowly creep forward and there wasn't much an AD based champion could do, they'd just progress steadily forward. I know Singed and Amumu were particularly common offenders of this, as well as any huge health stackers as AD is the thing that usually chunks it.
Essentially, with Thornmail a lot of champs got the capacity to tank all three champions, as it's armor and damage return was tuned for 5 people. If it comes back in any incarnation I can bet it's going to have a reduced effect in armor, return damage or both.