Probably because, from what I know, that's pretty much the highest level their digimon attained, except for Yamato and Taichi's digimon.
I've only watched the first Digimon Adventure series, here and there with the second, and pretty much none with the later ones. Though, if they all got to Mega, I think we could assume:
Sora: Phoenixmon Mimi: Rosemon Koushirou: Hercules-Kabuterimon Jou: I'm not sure... MarineAngemon? (So far from looking at some games and other evolution charts from long ago, I've been lead to assume this)
