Game was never played at Wembley at that point and Liverpool (the league champions) pulled out of the game first as they had a lucrative pre-season friendly arranged. The game should have been played at Anfield.
Liverpool were replaced by Man City who finished second in the league but by that time Sunderland had also organised their pre-season fixtures and Man City played Burnley (second division champions) instead.
The FA were not happy and moved the game to Wembley from the 1974 season to make it more appealing and stop teams pulling out.
Prior to the move to Wembley the charity shield was always played on the home ground of the league champions and often took place as a midweek game during the following season.
I'm nearly 47