Because they're the Romans!
The guys that conquered basically the entirely of southern Europe, north Africa and more!
They're the guys with the aqueducts, the baths, the forums, the temples, the arenas.
They're the guys that everyone else was trying to imitate, claiming to be the true successors of.
They're the guys whose names and words stereotypically end in "-ius", the guys who carried these long shields and made cool formations using them, the guys who had golden helmets and laurel wreaths.
They're the guys in Spartacus, the guys in Ben Hur, the guys in nearly every single biblical film (except if the film is about the story of Moses, in which case it's the Egyptians). They're the guys who Asterix and Obelix always sent flying through the air.
For the vast majority of the people posting here, the Romans heavily influenced they way they and their society live, and have done so for millennia. They have a huuuge influence in our way of life, our pop culture, even our own idea of what a Civilization
is.
This is regardless of how Rome was depicted in Civilization V. Rome, all in all, it's just... cool. And maybe those who are more into history see Rome with less of a spark. Maybe hardcore CivFanatics see Rome with less of a spark. But for the casual enthusiast, Rome is really really awesome.
And maybe it's just me, but I like to see really really awesome civs in Civilization games.