Other learned and more experienced members, especially current and previous railway employees, might be able to shed more precise light on this.
I can think of two completely unqualified reasons for doing so.. first, keeping them far enough from the engine to not be inconvenienced by the engine horn.. secondly on most stations, except terminals, the most facilities and amenities (food joints, water points, official offices, TTE offices, exit points or ramp/stairs etc.) are in the middle of the platform and maybe AC travelers are kept close to them since they pay the most fare..