Source: https://www.blockchain.com/charts/utxo-count?timespan=all
I think it's just because of the price, with 1btc at 1$, people will pay with blocks of 100 or even 1000btc and since there is only a limited supply of btc, you can only have few UTXOs. But with 1btc at 10 000$, people only need to spend small parts of these UTXOs each time. So BTCs are granularizing in several new UTXOs of small amount of satoshis.
If the price fell down to 1$ again people will need to consolidate several of these UTXOs to only pay 1$ in btc, and there will be less UTXOs.
In brief I think this chart is the inverse of the average amount in satoshis of UTXOs.