IronRuby has been out for 4 weeks, IronPython for 4 years. Developing an IDE takes months, if not years. When exactly where they supposed to squeeze that in?
Also, I believe the IronRuby team is smaller than the IronPython team.
There actually is a Ruby plugin for Visual Studio produced by SapphireSteel. It's called Ruby in Steel. Unfortunately, they currently only support MRI, YARV and JRuby. They did have IronRuby support at one point, but they removed it, because a) none of their customers actually used it, b) IronRuby was still changing faster than they could adapt and c) some of the IronRuby developers announced that Microsoft is considering developing IronRuby support for Visual Studio in the future and SapphireSteel didn't see much business sense in trying to compete with Microsoft.
Also, Visual Studio is not the only IDE on the planet. MonoDevelop has an open bug for IronRuby support, for example. And I'm pretty confident that it wouldn't be too hard to add IronRuby support to NetBeans: it already supports JRuby, MRI and YARV.