Looks like it's because until now it was not available, since rr has gained both reverse execution and x86-64 support quite recently (now and end of 2014 respec

Looks like it's because until now it was not available, since rr has gained both reverse execution and x86-64 support quite recently (now and end of 2014 respectively) There have been other similar tools previously, but they were either slow (chronicle), unable to work on complex programs or only worked on VMs.