When Faceit came into existence, PugChamp died, and many people didn't want to play Faceit, so in-house pug groups were formed. Faceit was not a good product, so people stopped pugging on it and it got to the point where way fewer advanced pugs were happening per day on Faceit than PugChamp previously had. Eventually Faceit died. When Faceit died and PugChamp became the standard pug service again, many people realized that in-houses are funner because they can play with their friends and curate who is allowed to play with them. Also they don't get fat kidded as much. Some of the people who add to pug champ these days are kinda not fun to play with -- the sort of folks who don't get invited to in-house pugs, so I doubt there's much motivation to add to PugChamp and make it alive over playing in-houses. Also there's this idea of having a critical mass of players. When PugChamp was more alive there were enough players for two pugs at once usually (A pug and B pug), so fatkidded players had a chance to play more. Now there's only one pug happening on PugChamp at a time, leaving a good number of perpetual fatkids (who can now only really practice 6s in scrims and in-houses), and so I wonder if many low IM/ high open people don't really feel motivated to add up. IMO it's not healthy to jerk a player base around between platforms like this.
Just some background... not trying to propose a solution or blame anyone