The answer is simple, at the same clocks the 7700K is 5% faster. At gaming with a 1080ti @1440p a 4Ghz 1500X vs 5Ghz 7700K are still between 0-5% of each other in fps.
1500X costs almost half of the i7 quad. What do you think? i7 line up is completely deleted by Ryzen, want an i7 quad? 1500X is that chip with more or less the same performance for half price.