The number on the page is a bit misleading. If you comb their code, you'll see that the 8,838 is actually the number of subreddits that agreed to participate.

Calling it a flop isn't accurate either, though.

Sure, most subreddits don't care, but the largest and most active subreddits are overwhelmingly in support of the blackout, but they are also much more affected by Reddit's changes than smaller subreddits.