Optics and zeitgeist.

The Snowden leaks had too much of an abstract bogeyman. PRISM couldn't inspire critical mass outrage because, at the time, Big Tech was still genuinely useful and, besides, the Big Tech involved could be sidestepped with some inconvenience. Flock is neither directly useful nor avoidable unless you resolve to never leave your house.

Metadata tracking is, again, too abstract of a concept, too much a statistical what-if. Compare that with Flock which tracks your very countenance.

Most importantly IMO, Flock has a physical manifestation on which people can direct their anger. There is a "frontend" to actively subvert as opposed to a mere "backend" to avoid.