I believe, to my understanding, that the reason the radar does not cover as big of an area as in the 64 is 1) Most of the time it isn't needed and 2) Having the game process all the radar data caused it to be heavily laggy/dysfunctional when it came to hit detection and registration with a lot of players in game. Granted it would be nice to be able to see more, but it also isn't worth ruining the game when a lot of people get on a server.
On top of that, it is a little more modern and realistically correct.
I always think of it like the motion tracker in AVP2....you can only see when you get in range. It isn't like it is realistic that you can see from the farthest map corner to the other perfectly.
Especially when the dot is stuck out there and not moving towards you until you get to a certain point in closeness...it can become kind of annoying.