Granted I am nowhere near an Invite player but it seems pretty obvious to me that Invite is an extremely small division and due to a variety of issues (skill disparity + players quitting + small division) there's a small number of players that would actually fit Invite only PUGs.
Then you factor in school, work, players trying relationships, players playing in Europe/multiple leagues, players playing other games compounded with the small insular division and getting decent Invite PUGs going seems pretty stupid hard.
All that stuff is kinda true for low level PUGs, but there's also way way way way way way more players that can play in those so grabbing 12 people to play for hours on end is comparatively easy.
to add on to this, getting quality invite pugs is much harder than you'd think. there just aren't 12 players that are at the same skill level online most of the time, and this is coming from someone who's setup numerous inhouse pugs and constantly tells people to add to pugs.
also, i personally think if you need to take the effort to tell people to add to a matchmaking system, it shouldn't be a mm system, but rather a system like pugme/pugchamp where an infinite amount of players can add, because when you look at mm and infinite adding system on a pro/con weighting you have
mm: more efficient (pro), lower quality (con)
infinite adding: infinite amount of quality (pro), less efficient
the reason the quality of mm pugs are lower is because there's always a much larger skill disparity throughout the players in the server.
given the amount of invite players, invite pugs do not need to be efficient and that has never been an issue, as quality is much more important to invite pugs; since pugchamp, i've played a singular quality invite pug