Technical reasons. The reasons for why you can't get back into Sailfish after booting CM aren't 100% clear but if resources aren't properly isolated, I would guess that there is almost certainly some leakage going on there, where CM is overwriting/changing some in-memory OS resource provided by Sailfish. What you're doing is running one OS inside another but both OSs share the same kernel. This is essentially OS-level virtualisation and existing solutions such as LXC make extensive use of namespaces to ensure that the guest OS is properly isolated from the host OS, mainly for security purposes but it also helps to ensure correct behaviour.