When crypto was brand new, mining started off using CPUs. However, that quickly gave way to the creation of ASICs (application-specific integrated circuits), which are purposely designed and built for just crypto mining, and often one specific hashing algorithm (like the one bitcoin uses).

There was a big belief in the community at the time that the intention of crypto should be "one PC, one vote". The ideal network was secured by a shitload of people using their PCs to mine and each person would have a vote in the next block chosen. ASICs eliminated that possibility because they were orders of magnitude better at mining than even a crazy high-end desktop PC. Regular people couldn't contribute to mining power significantly anymore, it was all about who had the best/most ASICs. That's why although bitcoin is the largest cryptocurrency by far, it doesn't directly affect consumer PC components when its mining becomes more/less profitable.

Enter Ethereum. This shook up crypto in many ways, one of which was to make the network less centralized, by having a shitload of miners rather than a few with mega expensive mining rigs. Now I don't know enough about the technicals here but it was intended to be ASIC-resistant, and for that to happen it doesn't use just one single hashing algorithm. So you can't build a circuit specifically optimized for one type of math problem, you need to be good at a bunch of them. Basically you'd need an all-around better CPU. But for whatever reason, GPUs tend to better at crunching large amounts of numbers like you'd need for hashing algorithms. The analogy I've heard is that a CPU is like a fighter jet, and the GPU is like a cargo ship. The jet will get there much faster but because it can't carry much, the ship will get 10,000 tons of cargo to its destination much sooner.

Of course, it didn't take long before people started buying a bunch of GPUs and hooking them together for mining. But at the very least, if you wanted to have 10x the hashing rate of the best GPU available, you had to spend ~10x as much on GPUs. Compared to bitcoin, where 10x the spend could get you thousands of times more power.