Because Scotty asked them to..

GM in general wants to shift their focus to SUVs.

The Aveo was a horrible vehicle. From what I can remember renting them, they weren't exactly quiet on the highway, they were underpowered, and I don't think they were terribly safe (of course, for some reason, I always ended up with a hatchback).  

They also don't hold their value or hold up too well, and start to need work at or even before 100k.  They just aren't a very well-built car.  You're better off buying a second-hand Corolla (even a 10 year old one) or possibly a Civic if you're looking for a good "cheap" car that's reliable.  I would stay away from the Aveo.