On Monday, I joined Software in the Public Interest (SPI). I’m currently a non-contributing member and have already applied for contributing membership.
Not knowing SPI is no good reason, at least if you are participating in Debian’s development, and you should really know SPI if you are a Debian Developer.
For me, personally, not joining the SPI would not make any sense. The existance of other organizations is no good reason. Look at me, I am also a Fellow of the Free Software Foundation Europe.
There is only one reason which seems to be a bit right: It’s hard to join SPI. The membership information is located at ‘About -> Membership’ in the menu. Maybe this should be made a top-level menu-item and a big link “JOIN!” should be put on the homepage.