[neilalexander]

Because there isn't "empty space" in the IPv4 packet header (or even the pseudoheader format from which TCP or UDP checksums etc are derived) to expand your new bits into. By breaking the packet format, you just invented a new network protocol that all of the routers, firewalls and middleware of the world don't know how to handle.

[db48x]

Yes, it’s true that any change they made would be incompatible with the existing software and routers and such. But nowadays everything can handle IPv6 just fine. All the upgrades and new software came out between 20 and 30 years ago, and is ubiquitous now.