Mining ASIC (Application-Specific Integrated Circuit) is hardware created exclusively for cryptocurrency mining. They are mainly used for mining bitcoin and litecoin; cryptocurrencies which their available coin poll has reduced very significantly. ASICs are efficient, but they can only be used to mine a singular coin. This makes you tied to that one coin - and your investment relies entirely on the future of that coin.
On the other hand, GPUs are great calculators for anything, and with a push of a button, you can mine a different coin. GPU is mainly used for mining Ethereum, Ripple and Monero. Now you have the choice - picking and choosing when to mine and which coin to mine. Miners use GPUs to provide them with this flexibility as the cryptocurrency market changes. This gives GPUs the upper hand and can greatly increase profits. And you can sell these GPUs in second-hand selling websites like eBay or OLX at a good price to recover some of the investment.
New blockchains like Ethereum can perform different types of calculations on the blockchain. Because of this, you are unlikely to ever see ASICs on Ethereum. There will always be a significant number of coins that are ASIC-resistant. The chances that one of those coins becomes profitable is higher than picking a singular coin and hoping that that one coin succeeds.
There is a situation where ASICs are beneficial. If you have a large equipment budget, access to very low-cost electricity, and you are OK at sticking to one coin for the long run no matter what - ASICs are by far the best choice for you. However, if you want flexibility and freedom from a singular coin, GPUs are the way to go.