For example, developing Ethash ASIC took long time and it's efficiency isn't very big against GPU (if you compare efficiency between GPU and ASIC with different ASIC-resistant algorithm).
To be fair the first bitcoin ASIC wasn't as good as they are today either. For example a single ASIC unit back in 2012-2013 was only working with 0.1 to 0.3 GHS speed.
Antminer E3 has 190 MHS
Btc-Digger (ASIC) had 150 MHS (released 2013)
Also you have to keep issues of altcoins in mind, for example in case of Ethereum the incentive to work on an ASIC was very little and instead the manufacturers were constantly discouraged from making such an investment since this centralized altcoin has always been threatening to change PoW algorithm, they even had a difficulty bomb that ensured it.
So it is understandable that nobody wanted to dedicate valuable resources to creating Ethash ASIC for many years.