What would be the point of developing with the Xamarin Platform on Linux? You'd only be able to target Android, not iOS, Mac, or Windows. In that case, one might as well not use Xamarin at all.
To target the full range of mobile devices, one has to have Windows, a Mac, or both anyway. So there doesn't seem to be much advantage to supporting development on Linux.
The point would be to develop on a platform that I like developing on. Either way, there isn't a single platform that you can use that will give you 100% of the available platforms...if you want cross-platform across Android, iOS, Mac, AND Windows, you are going to need a minimum of two computers. Why not develop as much as possible on the one you like?
To target the full range of mobile devices, one has to have Windows, a Mac, or both anyway. So there doesn't seem to be much advantage to supporting development on Linux.