I suspect it was just the conservative choice, mirroring the behavior of the predecessor browser extension (which also required deliberate user action to activate): http://paranoia.dubfire.net/2011/01/history-of-do-not-track-...

I couldn't find any explicit discussion of that aspect of the spec (https://datatracker.ietf.org/doc/html/draft-mayer-do-not-tra... is an early version), but I suspect that as noirscape wrote, it made consensus-building easier. Not that it mattered in the end, of course.