Why for Linux (I use Ubuntu & Mint) exist only very old binaries of the Mono (2.10.x instead of 3.4.0) and the MonoDevelop (2.6.0.1 instead of 4.2.2)?
That's how releases of Linux distributions (i.e. Ubuntu) work. Once Mono 2.10 is included in one Ubuntu release there will be only bug fixes applied on it but it will not be upgraded to newer versions until next major distribution release comes out. It helps to keep Linux distribution stable. There are of course some exceptions to this rule i.e. Ubuntu community maintains backports repositories which provide newer versions of some of the most popular applications. However Ubuntu 14.04 which is going to be released this month includes Mono 3.2.8 and MonoDevelop 4.0.12 so you can easily upgrade your OS to this version once it is out or you can try to build newer version of Mono from source code yourself.
Following text is a purely personal opinion (which have already brought me a downvote) based on my observations of Mono world: I have noticed some time ago that binary packages for newer Mono and MonoDevelop releases cannot be easily downloaded from their official project pages. Linux seems to be no more on the list of primary platforms for Mono project ever since a commercial company Xamarin has been founded by the developers that created Mono. They have also "forked" (if that is a correct word for what they did) MonoDevelop, improved it and they offer it now under a new name - Xamarin Studio. However it supports only Windows and Mac OS X because officially there is a weak commercial demand for Linux version but probably also because their CTO Miguel de Icaza likes OS X more than Linux. Who knows. But one thing is certain. They somehow forgot to update monodevelop.com website which still uses MonoDevelop branding but offers Xamarin Studio download instead. Nevertheless Xamarin is doing a great job and I really hope that they won't stop offering Xamarin Studio and Mono for free.