I think it's just crypto winter. There's also a lot more cards now than there used to be, so overall the supply is higher, which means lower rental income.
To that last point, I think most new players are going to be more familiar with Chaos Legion cards, which means they won't even know what Untamed/Alpha/Beta cards to rent. That combined with the size of Chaos Legion mean that rental prices are going to be depressed.
The card staking with the new land expansion should have an interesting effect. That will likely absorb a lot of the loose cards people have floating around and take out a lot of the supply. This should mean better rental income, but we'll see.
RE: Splinterlands Rental Income - Day 100