I could be wrong but the probable reason is once a demo is made and "tested" it has to go through the verification process with EA and then MS/Sony. So basically if we were to get a build closer to the final product we'd probably end up not getting it until the week of or after the retail version hits stores. Then people would bitch about no demo coming out soon enough.
Just my take....
EDIT: Nevermind. Already posted earlier as to demo process