Nonprofits tend have similar needs (clear path for collaboration);
Non-profits employ many technical people who (like all aspects of non-profits) are understaffed;
Non-profits don't have a lot of extra money to spend on software;
Non-profits understand working with volunteers;
Non-profits understand idealism (and often share our ideals);
Free Software organizations (e.g., FSF, FSFE, SPI, etc) are non-profits.