Numbers for sure. First attempt at forming inside we got about 21 people (2 parties and a couple people floating) then 6 and 7 people in my next 2 attempts so not even enough for 1 full party.
Also a mindset thing I'd say. Too many people sit outside and pass judgement on those who pug it with comments like "we barely cleared with a full 56 and on discord, you're attempting it with 20/30 and no discord..I give it 10 minutes and they'll be out" which leads to people thinking we can't clear it without being in a premade and anything else is futile.