During TPM Palpatine was actively doing things (the invasion of Naboo) while post-ROTJ he was trapped in a defective clone body he couldn’t do anything in. Also during TPM the government was a lot more greedy and corrupt then post-ROTJ. Every good and bad person contributes to the Force.
The reason it has to be balanced post-ROTJ is because it’s kinda directly stated that it is. Anakin brought balance to the Force. Even in TROS he says, “Bring back the balance, Rey, as I did.”