For one or more of the following reasons:

1. They play on low difficulty levels.
2. They are very good players.
3. They cheat.

When I just started EB, I loved playing as Rome and kicking everyone's behind. I've come to enjoy the more challenging factions though (Makedonia, KH, Nomads) on VH/H. When you defeat the grand army of Pyrrhus as Makedon with an army consisting of mainly militia and light troops, and only a small core of heavy cavalry and veteran pezhetairoi, on a challenging level, then it means something. When you rout the host of Antigonus and his heir with a rag-tag band of old-school hoplites and skirmishers, the feeling is far better than that of crushing a weaker opponent. To fight when you are on the brink of disaster and have (according to computer) an impossibly thin chance of victory, and yet prevail, that is what I find thrilling.

So yeah I like the "underpowered" factions.