User blog comment:Susanolivia/Fixed Width, Sidebar, and the Removal of Monaco/@comment-75.71.39.16-20101001171603/@comment-1154266-20101001212431

When you think about it, yes, it is favouritism in a way. But, remember that you can still choose Monobook as your own skin. It isn't available for wiki-wide default because it isn't compatible with many of the new features that they're bringing in and isn't officially supported; if it breaks, it breaks. So, it makes sense to have Monobook be a default skin on only one wiki to try and reduce the support work for those skins when you consider how they'll have a LOT of support issues to deal with as a result of bringing out the new skin.

While you could try and argue that they'd solve all those problems by not making a new skin, they've explained at some length why they're doing it.