Just about every single LCD monitor out there except for the new crops starting to come alive only support 60hz (except TVs). Therefore they are limited to displaying 60fps no matter what you set your frameRate to. Your dual link DVI or display port is also only capable of so much data as well.
Also Apple loves to point the finger at Adobe Flash Player for crashing browsers galore. One of the main reasons the late Jobs mentioned he will not support it on iOS devices. Lack of stability. Imagine a 120fps flash movie trying to run in Safari. Surely a sad Mac icon would turn up or you'd get the spinning beach ball of death soon after.
Being it's best to keep things even across all industries, 60fps is a pretty high target for Flash.
Is there any real reason you want to go beyond 60fps? Just out of sheer curiousity..
