Why wasn't the RADAR ever updated?

[PhantomHans]

So, I just have to ask, why wasn't the radar ever updated by most end users?

If I understand correctly, the F-4J and S got a pulse doppler radar.  But the F-4B,C,D,E,F,G, and N, other than those few later updated with modern radar by foreign users, did not get any updates.

 

As a side note, I understand the B,C,D, and N had a different radar than the APQ-120 but other than perhaps longer range due to the bigger dish I understand it to be quite similar?

I have no idea what the British F-4K/M had.

Seems kinda odd to me though as the radar is the one part of the jet that, at times, really seems to be lacking and hasn't been improved by the time of our E model.