Why are transaction fees so different? What changed that made Ethereum's fees go up in recent years?

When I try to search for why transaction fees between Bitcoin and Ethereum are so different, all I can find are articles from years ago when apparentley Ethereum's transaction fees were much lower than Bitcoin's.

Now, Ethereum's fees are so much higher. I paid $17 for an Ethereum transaction the other day (and I hear other people pay upwards of $100), meanwhile, recently I pay between $0.08-$0.16 per Bitcoin transaction.

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1. Why are Bitcoin's and Ethereum's transaction fees so different?
2. What changed that made it so Ethereum's fees used to be so low and now they're not?