Also, longer duration stays have an exponentially lower chance of getting an upgrade.
1 night - easy upgrade, very unlikely to sell that room
so why not give it out to someone who appreciates it.
2 nights - probably ok, especially if there are a few of each type available.
3+ nights, they will be banking on being able to sell the room in night 3 and onwards so even if there is availability they will be reluctant to give it out.
Also, I’m sure you’ve done your reading around the rest of the forum, but upgrades are waaaayy more common outside of the US. My travel is 50% UK, 40% Middle East, 10% Europe, usually 1 night at a time as I move around a lot, I get upgraded 80% of the time. 20% of the time it is a significant upgrade.