I suppose there was always some doubt with that the Cup Final might go to extra time, penalties, then you've got all the presentations and post-race interviews and chat .... it could have gone on until well after 6pm so it would be tricky to schedule the Derby not knowing what time the football might finish ..........
I guess it was just safer and more certain to put it on before the match ....... and that meant more than an hour before 3pm because ITV wanted to get on with the match 'build up'