As a gesture of goodwill perhaps ?
The same as somehow Lahore wasn't captured even though they were so close that they could see the Allama Iqbal Intl Airport , Lahore ... Why ? Because the cease-fire was to be announced the next day according to Indian version
... Do you know what sort of leverage would that have provided your country on the negotiation table afterwards if you had captured Lahore ?
I am talking of Haji Pir Pass captured in '47 ... You captured and returned it in '65 , there were no permanent changes there ... Rest assured it wasn't due to bona fide , we returned lands too ...
But still Pakistan fought in Rann of Kutch and retained 980 km2 of land , authenticated by British afterwards in meditation ...