It seems that the processes don't get cleaned up properly each time you stop the app with ps aux
in Terminal showing a large number of processes representing your app on the simulator.
I expect some kind of upper limit is reached with the number of processes / memory. (Indeed I have found that XCode will fail to build because some posix resource can't be found because there isn't memory to load it)
A machine restart clears all of these dead processes and allows room for new ones. For me, I only have to do this after about 2-3 weeks of not turning my computer off, but I suppose what resources you have and how often you use the Simulator will affect it.
If someone knows how to clear all these dead processes without restarting, that would be useful please, killall ...
doesn't seem to have worked for me, but feel free to add to this answer.