Because they don't always fill it during the off-season the rest of the year and because they don't have wide-bodies to use on the routes even if they wanted to.

The 752 is a great plane for the service in question. Very efficient for the number of seats, good range out of EWR fo rmost of the destinations CO uses it to and they've equipped it with a pretty good IFE system. I love taking the 752 across the pond on CO. Way better than the 76s and really about the same as a 772 to me.