I don’t think fiat price is as correlated as you may think. Majority of people who care about the fiat price are most likely holding in an exchange or ETF, and not really using the Bitcoin network. Therefore, mining fees don’t affect them since on the exchanges it’s all pretend Bitcoin anyway. Only if they self custody would transaction fees come into play for them.

Aside from that, I think it’s just a supply and demand thing? I’m no economist, but my understanding is that transaction fees will go up if there are more pending transactions than can fit in a block. So if there aren’t many people trying to make on chain transactions, there’s no reason to pay high fees