To be fair, another reason could be that the mainstream apologists see the issue as so well settled that the very act of debating it would be to lend credence or legitimacy to the other side, which is in their eyes, destructive, heretical and only currently held by a small few.
Also the sedes have made a cottage industry out of going into painstaking meticulous detail to craft arguments from all sorts of Church documents to support their position. They have written many legalistic volumes on the topic. A mainstream apologist and especially a busy Professor like Hahn would have to take months out of his schedule to meticulously research and study in order to refute a position that only .001% of the Catholic population even takes seriously and, to them, most likely is, on its face, absurd.
After all, what kind of god abandons his church without a head for 50+ years and allows them to drift into oblivion. It is akin to debating atheists. There are very few atheists and their claims are absurd on their face, but require months of study to properly academically and technically refute claim by claim.