Sessions with the D&D Club at school get cancelled if they conflict with another event, like last week, when most of my players were also in our school musical.
At home, the cause is typically that the games are only a priority for some of us, but I don't want to kick people out because they're friends and family. I would dearly love to be part of a campaign where everyone else was as invested as I was; I've never really had that, to be honest.
I put a TON of work into every session, usually as DM, but also on the rare occasions when I get to be a player, and it's a real bummer when someone blows off a game.