because it can mine efficiently bitcoin much better than any gpu, for example

one asic antmienr s9 is running at 14TH, the most powerful gpu only 1gh, you can see here that one single asic is 14,000 times more faster than a gpu, while consuming only 14x of a gpu

1000 times more efficient, for the best gpu right now 1070 nvidia and 470/480 amd are the best choice