XCode 7. iOS simulators missing and not installable
One possible issue is that there may be old leftover simulators installed, which are not compatible with the new XCode, and their presence causes the whole Simulator to fail. To get rid of them, delete the simulators in /Library/Developer/CoreSimulator/Profiles/Runtimes (for me there was an iOS 7 simulator there which was the issue). Relaunch XCode after this.
Then try to run the simulator from XCode->Open Developer Tool->Simulator. Assuming this succeeds, verify that you have the simulators under the iOS Simulator menu Hardware->Device->Manage Devices… – if not, you should be able to click the + in the bottom bar to add yourself some iOS 9 simulators. Relaunch XCode again and they should show up in the menu.
In my case was with Xcode 8.2, what it worked to me was changing Deployment Version:
After setting another different than 10.0 then appeared back all simulators again.
Have you tried going to Xcode > preferences > downloads and trying to re-download the simulators you want?
