GPUs are really good at both running the algorithms to 'create' coins and being stackable. That is why they're used. However, it isn't their primary purpose which is why the crypto market's demand for them is extremely fickle.

ASICs or "application-specific integrated circuits" are designed to do exactly one thing and do it better than any other possible approach but not being usable for anything else. Bitcoin ASICs have been around for awhile and their existence made GPUs worthless for Bitcoin mining. ASICs for Ethereum have not been fully realized as of yet and with the gigantic surge in Ethereum's value (as a result of Bitcoin's giant surge), GPUs became a target for that market.

As an aside, coins like Ethereum were designed to be "ASIC-resistant" which is why the development of an ASIC hasn't been terribly fast. Though if you just think about that term of what "ASIC" refers to, you recognize why "ASIC-resistance" is an idiotic concept at the outset and it's just a matter of time. The only thing they're "ASIC-resistant" to are the original methods and techniques used to create the Bitcoin ASICs. Either the value of the coin will dwindle below the value in mining it with GPUs or an ASIC will arrive.