There are so many things that went wrong there, it wasn't just one thing.
- the police assumed that less people would come because of Covid, when actually, may people who do not go every year went this year since they couldn't last year
- the various government ministries spent a lot of time trying to work out how to handle it this year, causing a delay in the actual planning in Meron
- the fact that Lag B'Omer was Erev Shabbos meant that more people went up by night instead of waiting to go later in the day
- the petira of R' Kupershtock meant that there was no one experienced enough with coordinating the various organizations involved
In short, there were many parts to this tragic event - so many things just waiting to go wrong! The normal barriers placed even a millimeter closer together, a mistake about whether the people were trying to enter or leave, not enough garbage cans causing people to drop bags and bottles as they squeezed through a narrow space, leading to someone slipping ...