Often happens to me, I create a new project (currently iOS 11 because we always pick the max version), do some testing and change my Xcode to an older one (which doesn't support iOS 11) and I get confused because no simulators show up.
We should do the same this we do for device, show a greyed out section;
Try to lower the 'Deployment Target' to see older simulators
Fixed in version 188.8.131.521 (master)
Author: Vincent Dondain
Commit: 89189e5c488ae9fbbb4c65d5ea257a88a08a9879 (xamarin/md-addins)
Included in Commit: 318f13004b9dacbb53ce6fb3be1e3dead8103bf8 (mono/monodevelop)