Frankly I'm supprised this hasn't been done sooner. I've always used monobook and I'll be sad to see it go but it has been here for a long time where other companies would have cut support for it ages ago.
I'm sure wikia has the stats on who uses monobook and that was used to form the decision. If I were them I'd weigh that against the estimated time a project like this would take, bearing in mind that the skin may need to be worked on again in the future. Honestly the reason it's been here this far is probably because something like this hasn't come up before now