"Remind us again why the IETF didn't make IPv6 backwards compatible"
Because, for the Nth time, IPv4 was designed with no provision for forwards compatibility. The address length was set at 32 bits, and that's all. So from the very beginning, longer addresses required a new version. (Actually, of course, there was a version number in the first place. So you could say that the version number is the forwards compatibility feature in IPv4. But it doesn't solve the problem that 32 bits is 32 bits.)
IPv6 wasn't designed by theoreticians. It was designed by people working with IPv4, DECnet Phase IV, Novell Netware, and Appletalk, to name but four. Plus people working with OSI and DECnet Phase V. Plus people working with IBM SNA. It couldn't be backwards compatible because of the design flaw in IPv4, so the model from the beginning was co-existence. That's still the model, and it works well, except that now the plan is to provide IPv4 as a service over IPv6, when needed. (Except that isn't a plan, it's deployed already by various ISPs.) Anyway, 25% of the Internet is now IPv6, and most users don't even know it. Apparently most of the commenters here don't know it either.
I don't really understand why people bleat about the issues with IPv6 (yes, of course there are issues, but there are plenty of issues with IPv4+NAT) instead of just running it and making it work.