Because the micro-pulse of 6/8 is different than 3/4. 3/4 is naturally emphasized as "1 2 3 1 2 3. In contrast, 6/8 is naturally emphasized with a micro pulse of 1 2 3 4 5 6, which produces a strong pulse on 1, a slightly weaker pulse on 4, and a macro pulse of "1 2. (Think 4/4 with triplets). You thus get a much different feeling from 6/8 than 3/4.