A lot of developers don't trust MS even if they have some really nice tools and a huge platform. Microsoft has a reputation for killing off development platforms when they start thrashing around seeking relevance in some new way. With the shift to Win 8 for example, Silverlight and XNA were axed with little more than the promise that critical bugs and security issues will be fixed. If the platforms were open sourced the community could pick up the slack. The Mono project did that for XNA care of Mono Game. No such luck for Silverlight as Moonlight is dead as well.