Because, really, why should I expect a newer version of a mostly-text document to be backwards-compatible with an older version of a mostly-text document?
You’re doing a helluva job there, Apple…
Why proprietary software sucks in eleven words (and one screen grab)…
Andrew Currie 
AC:
It’s no better in the PC world, either. At work, they’ve tweaked our Office 2007 apps to save into the 97-2003 format since most of the world is still using older versions of Office. The new .docx and .xlsx formats aren’t backwards compatible, which means I get a warning about saving into an older format every time I set up a new doc or spreadsheet and save it.
Ed
That’s a good thing, as your company might be spared the charge of running illegal software.