[msbarnett]

That would have completely destroyed backwards compatibility for a large number of Mac OS users' files.

Remember that you could run Mac OS Classic apps on the first few iterations of OS X, and even after you couldn't any more, Carbonized/Cocoized OS X Apps still could open old files you created, many of which heavily used resource forks.

[duskwuff]

AND many Carbon apps, even ones which wouldn't run on Classic Mac OS, would still load resources from a resource fork.