ASIC Miners

I just ordered my first asic miners for another algo.

Now I understand a little more about an asic and how they operate, which isn't much different than GPU mining.

Since RVN is asic resistant and can only be mined by GPUs, I am now curious what is it about asic miners that developers do not like or that an asic is capable of, that they make the algo's so they cant be mined with asic's?