Because we need more stimulation than guys do!
I can orgasim fast if i want to, probably faster than a guy can, but there never good fast. The longer it takes the better they usualy are lol.

Mainly because men go into the act of sex already aroused (ie:erection). Sometimes I can orgasm quickly if I'm already aroused when intercourse begins, sometimes I need some stimulation before. Also I find relaxation has a lot to do with it. Sometimes women feel pressured to orgasm to make their man fell like "he's the man" but in my experience trying to force orgasm just delays it or makes it impossible.