Because it's a consensus rule (enforced by every fully validating node), it would require a backwards incompatible change (hard fork) to be implemented by every

Because it's a consensus rule (enforced by every fully validating node), it would require a backwards incompatible change (hard fork) to be implemented by every single miner, exchange, wallet and merchant. Only soft forks (which are backwards compatible) can be rolled out with a 51% miner vote. Bitcoin Classic is trying to gain support for a hard fork with 75% miner vote, but in reality this would also require every exchange, wallet and merchant to pledge their support, or risk creating a network split with two separate currencies.