why is gaining partner debited and sacrificing partner credited when there is change in PSR among partners during death of a partner during the year? 

Dear Student,
Here is your answer,
Gaining partner is debited because he is the person who will gain share of profit in future and for that purpose his capital account should be debited or deducted with the amount. Similarly sacrificing partner is the person who will sacrifice his share of profit in future therefore he must be compensated at this time so his capital account is credited.
Regards.