AZ, the problem is that there are many issues the gedolim have commented on, e.g. takanos for chasanos, and there has not been uniform compliance. Many people have reservations about this. It is one thing to SUGGEST that one should try to date within the age range. But to MANDATE marrying within the age range, i.e. ONLY considering shidduchim within the age range, gives many people pause. It is social engineering, and requires a level of unity and maybe even ruach hakodesh, if not outright nevuah. No one wants to be the guinea pig.
There have been other suggestions as to how to pull this off, e.g. have boys coming back from E”Y at an arbitrary cut off date, regardless of where they are in their learning (and what if they don’t want to start dating that early, anyway?). The buzz gives many people pause.