Why 110m hurdles for men and not 100m?

Answer 1

Because the hurdles for men are set at different distances from the starting line than for women. For example: The mens hurdles start at the blue line, so the womens would start about 2 inches away at the yellow line.

Additionally, elite men's stride length would not allow them to hurdle at the distance that womens hurdles are set at; it's simply too short.