1. Difficulty increases much faster for ASIC coins.

2. ASICs are harder to use (either you need a separate building/warehouse or you need to tolerate a jet engine in your home).

3. GPUs have much higher resale value and have value that is independent of the cryptocurrency market.

GPU mining is therefore much less risky, which is why it has a lower return on paper. It's a much better investment, in my view, than ASIC mining.