Copyleft means "you can redistribute the work, as long as you stick to the exact same conditions/license". This way the licenses protect the rights granted by the license for everyone. This means that by default two copyleft licenses can only be compatible if they are exactly the same.
In order to make two copyleft licenses compatible the author of the license have to add some explicit "compatibility clause". The most used copyleft license, the GNU GPL, does it with the "or any-later version"-clause. This kind of forward-compatibility makes a lot of sense because normally you want to be able to update to a newer version of the license which most of the time improve the wording and/or take new legal and technical challenges into account.
On the other hand backward-compatibility would make all the improvements of the newer version void because everyone could ignore it and go back to the older version. I would say that this is the main reason why backward-compatibility doesn't make sense. On the other hand forward-compatibility makes a lot of sense and I'm surprised that Creative Commons doesn't have such a clause.