Because Debian does not meet the strict requirements of the FSF. It includes non-free blobs in the kernel and the FSF claims Debian "steers" users with recommendations for installing non-free plugins or codecs. Some "contrib" packages, while free themselves, exist primarily to load separately distributed proprietary programs. There are also references in the Debian documentation and official channels that suggest obtaining non-free software for functionality.
edit: typos
While I think it would be too hard for most people to be completely free of proprietary software, atleast he is practicing what he preaches. It is a nice goal to someday get there, but I don't think its realistic at the moment.
Kind in mind, though, he is 72 and I don't think he even codes anymore. His computer use probably only consists of mostly Emac (for all text based work) and a web browser (which I read he has a very particular method that involves something similar to wget, lynx, and konqueror). His computer use is very light (I imagine) compared to many Linux users.
While I aspire to and appreciate what the FSF advocates, I don't see a realistic path for myself as a Linux gamer. The proprietary firmware limitations alone would keep you on 2015 hardware.
Source: https://kottke.org/15/05/how-richard-stallman-does-his-computing