I just took out money this weekend and was surprised to see a mix of tens and 20's.
When I was working in a bank, a group of efficiency experts came through and one of their suggestions was to use all the drawers of the ATM and put 50's in one of them. Our policy was all get 20's. Period.
Their reasoning was you would not have to refill it as much. If somebody wanted $100, it would give them 2 x 50 instead. Two bills instead of five.
Our reasoning was no, because we counted the money in the ATM every day anyway, so that wasn't an issue, and we referred them to the many instances where somebody put a tray of $50's where a tray of $20's should be, so when somebody asked for $100 they got 5 x 50 instead.
And if one tray jammed (which happens all the time) the machine could still dispense normally, not having to say, sorry, $40 is not a valid amount.
All that being said, there are banks that do trays of $5, $10, $20's $50's and I know of one that does $100's. Not sure it it's still this way or not, but at Broadway Pizza, think it was 28th and Broadway, they had two ATM's next to each other. One did $20's the other did $50's. Our office was right around the corner, I got slices there several times a week. The $20 machine was always empty, they couldn't get the people there often enough to keep it stocked. The $50 machine nobody wanted to use. They hated it when you stopped at the machine, got a $50, then walked in and bought a slice for $3 with it. Well then, fix the other machine, OK?
It's just much easier for the bank to track if all the trays are the same. There's way too many things that can go wrong if you load each tray with a different value bill. Some banks are obviously willing to deal with those issues, but for the most part, banks just order "ATM quality" 20's and that's that.