Thank you for this question.
The Mono implementation of .Net framework supports the default implementation of WinForms. However, some methods are not implemented or behave different than in MS WinForms. Our controls add rich functionality and visual effects on top of the WinForms platform and to achieve that, they need access to the underlying operating system. This is done by using pinvoke calls. There is no way or it will be extremely difficult to implement all those calls with Mono. That is why RadControls for WinForms is not available for Mono.
Please note that since 2011 the Mono WinForms implementation is no longer supported and updated.
If you have further questions, do not hesitate to ask.
the Telerik team