The female's genes are most likely to get passed down many generations if the males she mates with have genes that enable her male children to survive long enough to reproduce. Once a species gets into a state where females make a habit of eating the males during copulation, it can't readily evolve out of that state, because a female that is less aggressive will mate with males who can't succeed at mating with other females, leading to that line dying out. Sexual cannibalism is a local maximum.
So if at any point in the past there were evolutionary pressure that made cannibalism beneficial to the genes, we would expect to see it stick around until there's sufficient pressure against cannibalism to allow selection away from that local maximum.