Because 0 A.D. is still in development, and what the full release of Part 1 Empires Ascendant will look like still isn't known quite yet. The decision to add the Successors and Mauryans, split the Hellenes into the Athenians, Spartans, and Macedonians, and the Celts into the Britons and Gauls, were both rather sudden changes. If I am not mistaken, some work has gone into triggers, which are a major roadblock for campaigns and are technically planned for Part 2 Empires Besieged. With that development, I honestly wouldn't be shocked if campaigns for Empires Ascendant were revisited eventually, which would probably lead to the inclusion of the Thebans.