Its actually not hard to accomplish if you either perform a hardfork to a slightly different algo in a random way every year and use asic resistant algos. This can even be done in a automated way where there only needs to be one hardfork. There is a tremendous amount of investment and lead up time to creating ASICs where you only need to stay ahead to make their implementation impractical.
You either manage that in a somewhat predictable way which ends up still being subject to being dominated by specialized hardware or you do it in a completely freeform way which is a lot of work and is a huge security risk if you make a mistake.