Because DirecTV doesn't want you using any products that THEY aren't getting paid for (kind of like Nextel.. they think they're GOD!)

The "dataport" on the back of the Directv receivers is rarely compatable with anything (other than Tivo or other DVR's offered by Directv). I'm not sure if this website will have anything for you, it's primarily for problems associated with ReplayTV & DirecTV. But it will at least help explain your problem.

http://www.patersontech.com/Translator/TransMain.aspx