The problem is that replacing the helmets after each play or major impact would take time, and the clock doesn't always stop after a play during football. If we stopped the clock to replace a helmet, we open up the possibility of a team using this to slow down an advancing offense, or by an offense to gain a short timeout in a sense.
Basically it just doesn't mesh well with the flow of the game.
Basically it just doesn't mesh well with the flow of the game.