HWH has 23 platforms shared between ER and SER.
Platform 1-13 exclusively for ER trains.
Platforms 14-23 for SER trains.
The reason as clarified by Bishwanath sir anyway is due to this shared platform policy employed in HWH.
Another...
more... reason is that at any given instant there are always 7-8 local trains either entering or leaving the platforms of HWH. Priority has to be allocated to the office time local trains and long distance trains face the brunt of this policy.This is actually the primary reason why HWH faces a shortage of platforms!
Another reason is that though HWH does have 23 platforms, most of them are incapable of holding long distance trains which are usually of the length of 24 icf coaches!