Because it contains new features. That's really all there is to it -- what would an older version do with them?
@adluf – that's the way it is and ever was.
Instead use export to IDML.
Btw. Do you mean version 6 vs. version 7 ?
That would be CS4 vs. CS5.
If you mean CS6 vs. CS7, I have to say: There is no CS7. It's CC v.9.3.0 (the latest bug-fix iteration)
If you get a warning message that states InDesign "CS7", I'm quite sure you are not updated to the latest bug-fix version of CS6 v8.1.0.
For CC members there is a new "Simple Save Back"-Service, if you are online.
But under the hood, it's doing an IDML => Indd- file conversion.