Thanks for the question, Anon.
Internal Northern conflicts and Stark infighting, it seems. At some point during the reign of Daeron II, the Skagosi rose in rebellion against Winterfell, and suppression of that rebellion cost the life of Lord Barthogan Stark (called “Barth Blacksword”), as well as thousands of others. We don’t know if this rebellion preceded or came after the First Blackfyre Rebellion, but either way, its existence could explain in part why the Starks and their bannermen avoided this civil war, while having participated in the Dance of the Dragons. Thousands being killed means thousands more engaging in the conflict. Have the rebellion end before the Blackfyre Rebellion, and the North is too busy patching itself up to spare its legions either for Daemon or Daeron; have the Skagosi rise up during or not too long after the Blackfyre Rebellion, and the Starks may have been too distracted by the discontented grumblings/open warfare of the Skagosi to risk sending their best men south of the Neck.
What’s more, Yandel tells us in TWOIAF that “Rickon [Stark]’s death outside of Sunspear in one of the final battles [of the Conquest of Dorne] was lamented in the North for years to come because of the troubles that dogged the reigns of his half brothers”. There seems to have been a Stark succession crisis after the death of Cregan the One-Day Hand, with Cregan’s sons from his final marriage jockeying for the lordship of Winterfell (and two of them marrying the legitimate Stark heiresses). Again, depending on what was happening within House Stark, any of the wolf brothers might have been reluctant to lead troops south: if the Lord of Winterfell led his troops to fight for either side, one of his brothers might have taken the chance to lead a coup against him, and the lord would return to find the North hailing the Stark in Winterfell as the true Lord of the North.
(Incidentally, on the Redgrass Field podcast ep, @racefortheironthrone referred to this Stark infighting as a “cold war”, which had the best dry, mocking laugh I’ve ever heard from @warsofasoiaf.)
The Queen Regent (NFriel)
I forgot I made that joke! Ah, good times. Good times.