My guess is: they are letting the privacy nuts (and I am one) figure out the kinks in the system first before unleashing it on the general public

1) It breaks some webpages, so they probably only want to have it where it's relatively clear that users will prefer their privacy over not having breakages.

2) It pisses off webpage owners, as it hampers their analytics and probably ad revenue. If webpage owners are too pissed off by Firefox or simply don't make enough revenue from it, they'll stop testing/building their webpage against it, which leads to broken webpages in this way as well.