The reason for this is that the Ubuntu development team has decided to drop the Mono-dependent Banshee music player and go back to RhythmBox which was used in earlier releases. Once Banshee is removed, the main reason for the inclusion of Mono goes with it.
Mono, for those who are unaware, is an attempt by Miguel de Icaza, co-founder of the GNOME desktop project and a former vice-president at Novell, to create an open source implementation of Microsoft's .NET development environment. Mono has attracted a fair share of controversy as many in FOSS circles fear that it may pose patent problems
Banshee is being excluded as it does not work on the ARM platform, one of the platforms which Ubuntu is looking to conquer.
When Banshee was included, one of the reasons given was that it took up less space than RhythmBox and would help in Ubuntu's bid to keep the base distribution small enough to fit on a single CD.
That is no longer an objective and the image of the next release, Precise Pangolin or 12.04, is likely to require a DVD for burning as it will be in the region of 750 MB.
Along with Banshee, there are two other Mono-dependent applications included in Ubuntu - Tomboy, which is used for note-taking, and gbrainy, a brain-teaser game.
Both these applications are likely to disappear in the next release as well.