All dragons have sworn never to return, after the Homeland Wars that nearly drove them extinct fighting over the territory, that tore the land asunder in the sites of the worst battles. The survivors called an uneasy truce with one another, and one of the terms was an agreement that they could never again set foot on this place - and they all know that the dragon to break that agreement will singlehandedly end the uneasy, relative peace that exists between dragons - and it may not peaceful per se, but it's not what it was - but will also be personally slaughtered by every other dragon upon its return.
The dragons communicate with the smaller beings - or perhaps use magic? - on a regular basis to ensure that no dragons have been seen on this continent. All secretly long for their homeland, but most love it too much to be the one to break that truce.
(Plot hooks: "All except one..." or "But the humans who captured Skittish the Dragon have no such sentimentality..." or "But then a long-dormant egg hatched and a dragon who had no knowledge of the truce or the ways of Dragonkind was born there...")
If we don't want to rely on fear and sentimentality, the Great Dragon General-Mages may have also cast an eternal ward as the last of the dragons left, preventing the dragons from crossing. ("Until, after millentia, the spell began to weaken...")