The reason is that it would severely dent the Premiership and Nationwide conference gate receipts for Saturday mathes which is when almost all games are played- 3-5pm on Saturdays is THE time. There is a football fanzine called ‘When Saturday Comes.’
A good proportion of the expected gate at the Saturday matches might choose to stay and watch a blockbuster match on TV.
Football leagues do not like Saturday afternoon games being shown live. Fans may choose to stay home and watch the televised game rather than going out to see their local team. This is particularly a worry for lower leagues and smaller teams. So the contract with the TV companies expressly forbid showing any of the game live. This is why you get the daft Saturday afternoon programmes of footie pundits commenting on games’ live action off a studio monitor, but not showing anything to the viewer at home.
If the TV company wants to show a game, it is usually rescheduled. The exception to this is cup days when no other games (or few) are being played.