The audience and the posters are the content, not enough people want to start posting and commenting on a dead reddit clone to get it off the ground to entice more people
And also, there was less competition and a lot of lazy market leaders. Digg v3 facilitated reddit's rise in a week the same way that Skype's negligence and TeamSpeak's technical focus made is really easy for Discord to take over.
I think user moderation is where the opening will be. Reddit as a whole is pretty heavy handed in banning anyone who says anything wrongthink, or worse, actively bans users who participate in subs they don't approve of.
You had Voat spin out but it had the same problem, with a vastly different opinion of what wrongthink was. It didn't last long.
I think the between that, the unpopular API changes, new Reddit that nobody asked for, and now paywalls, there is an opening. One challenge is that it's hard to actually make money from the idea, which I guess is why there haven't been a lot of entrants in the first place.
