It has to do with the fact that GPUs can use less power and produce more heat.
Traditional CPUs – including high-end ones – may be overkill for mining some cryptocurrencies, and some countries (Russia and China) have outright banned their use for mining due to the extreme energy consumption of bitcoin mining overall. But ASICs tend to require a lot of electricity which means low accessibility in regions without an abundance of either cheap or free-to-use power sources.
What makes GPU graphics cards such good candidates for cryptocurrency use is low power usage combined with high processing frequency, coupled with things like cooler operating temperatures generated by customized heatsinks.
Using GPUs for mining Bitcoin saves you the electricity that would be needed to power the CPU. During Bitcoin transactions, the Bitcoin protocol splits up the workload so not every transaction goes over one miner’s line. So it can share among many miners who are connected to Bitcoin. Since your GPU is much faster than your CPU, you will get more Bitcoin if you use a GPU to mine Bitcoin.