After installing Reader XI, why can't Acrobat be set as default PDF handler?

I've tried everything I know, including a clean boot.  Every time I try to set Win7 to open PDFs with Acrobat (Version 8.3.1), it sets it back to Reader XI. 

From inside Reader/Preferences/General/Select Default PDF Handler, Reader XI is the only option.  Acrobat isn't listed.