Well, I think the 1.5L engine size has to do with other country's tax codes and stuff. So for a global engine, that makes sense to make it fit in all over the place instead of something just for the US market.
But I mean, if Mitsu is going to put in an optional engine, the 1.5 makes sense to me because it's a more modern powerplant what with a turbocharger, direct injection, and other stuff I can't remember. Plus, the 2.0 and 2.4 engines are pretty damn old, dating back to 2007 I think or maybe a little before and they're also not very small physically so it would take more effort to cram that into the Mirage's small engine bay than the tightly packed little 1.5L turbo.