It’s not unique to these spiders; a similar thing evolved with ducks. I think it’s a trade-off in species where the males tend to be, aggressive in their mating attempts. More complicated female sex organs makes it more difficult for the males to mate, which dissuades them, which means the females are spending less energy dealing with and recovering from the males mating attempts. Which then puts an evolutionary pressure on the males to develop longer/more complicated organs to match.