If the newest version of a secure communication protocol includes some way to negotiate down to an older version, that opens the door to downgrade attacks - you risk ending up with a protocol that, in practice, has all the vulnerabilities of both versions.
You can work around this by having downgrade protection, and TLS 1.3 has this out of the box, it was also added belatedly to TLS 1.2 (but obviously the problem there is, you can still downgrade whenever either client or server knows TLS 1.2 but doesn't have protection yet)
In TLS 1.3 the downgrade protection works like this:
If I'm a TLS 1.3 server, and a connection arrives that says it can only handle TLS 1.2 or lower, I scribble the letters "DOWNGRD" (in ASCII) near the end of a field labelled Random that is normally entirely full of random bytes.
If I'm a TLS 1.3 client, I try to ask for TLS 1.3 from the server when I connect, if instead I get a TLS 1.2 or earlier reply, I check the Random field, and see if it spells out "DOWNGRD" near the end. If it does, somebody is trying to downgrade my connection, I am being attacked and can't continue.
This trick works because if bad guys tamper with the Random field then the connection mysteriously fails (client and server are relying on both knowing all these bytes to choose their encryption keys with ephemeral mode) while older clients won't see any meaning in the letters DOWNGRD near the end of these random bytes - so they won't freak out.
You might worry: What if somebody just randomly picked "DOWNGRD" by accident for a TLS 1.3 connection ? If every single person in the world makes one connection per second, this is likely to happen to one person, somewhere, only once every few years. So we don't worry about this.