Probably the same reason Autodesk changes dwg format every 3 or so releases - to get more upgrade customers! ;-)
They say it is to add new features..., but I have my doubts. Granted some of the newer features get hosed when saving to an older version so, maybe the older file structure doesn't support it...
Windows 7 x64, Precision T3400, Intel C2Q @ 3 GHz
8 GB RAM, ATi Radeon HD 3870 |