Why Epirus instead of Illyria?

[Tellos Athenaios]

If only for the fact that Epeiros in 272 BC was actually one political entity as opposed to the independent and in-fighting Illyrians?

And actually Epeiros is pretty unique insofar as that it is the Westernmost 'true' Western Greeks - relatively tribal compared to the other Greek factions, a different dialect from the lingua-franca that Attic had just about become, and most notable - those Illyrians you speak of a partly just Epeirotai by 272 BC, members of the league.

Finally at 272 BC Epeiros was the big guy of Hellas: bigger than Makedonia, and far bigger than any of the Leagues. Illyria would never become as significant during the game's timeframe as Epeiros was right at the start.

EDIT: You may want to keep in mind the various Illyrian ethnicities for Epeiros also; and note the existence of the Syrakosios and similar ones as well.