What resolution is required for synthetic vision?
AFAIK, mainstream CRTs are capable of AT LEAST UXGA (1600x1200). I have seen reports of people using a CRT for 2048x1536 @60Hz.
I believe the REAL answer is that CRTs are much heavier and require significantly more power (and generate more heat) than LCD displays. There is little chance that someone would design a "new" avionics system based on CRTs, if LCD is an option.