My “other” vote is due to completely forseen work-related absences (specifically I was on a work trip so couldn’t make it) - we knew a month ahead of time we’d miss that week.
My more normal reasons are illness and work emergency.
We have a largish group with three campaigns going - each meets every third week (mostly). Two campaigns have 6 players and one has 4. Some players play in all three, others in just 1 or 2. We’ll play as long as we have 3 players - the PCs of absent players fade into the background. The two larger groups have never cancelled due to lack of players.
We have skipped weeks due to holidays or DM travel plans. But that is known well ahead of time.
Player sickness has been a repeated reason for cancelled sessions in the past few weeks. But at least the one I ran on Monday happened (though things were delayed by Discord acting up for the first 30 minutes).
I voted one or players were sick. To be honest, I didn't' know why they couldn't make it and I never ask. The reason is really not important to me. To be clear, I do care about their well being and I assume they're ok.
It's just that if you can't make, then you can't make it. No big deal. The reason doesn't matter to me.
Since COVID, we pretty much have a policy that anyone who is sick with something communicable should stay home. A few weeks ago we had one person who was exposed to COVID and elected to stay home the same week where two others cancelled because they both came down with something (they lived together). With three players down, I decided to cancel.
I (the DM) found out the day before, that there was a leaky water pipe in my basement storage, so I had to clean out a lot of damaged cardboard boxes and contact my insurance company.