Zck's answer points out the fallacy of the "why" question, but in gneneral I would surmise that co-evolution of sex-linked traits like this are a harder hill to climb than simple evolutionary adaption of the sperm alone. Developing a "program" that is not much more than "when you hit body-temp it is 'go time'" is less susceptible to external forces or attempts at subversion by either other sperm or the female reproductive tract.
