I wouldn't say its a very 'low' design site as much as it is a very 'simply' designed site. I find HN to be very easily browsed and wouldn't have it any other way. The simple design shouldn't have any bearing on the number of female visitors to HN.
disagree, there are the blatantly obvious reasons as stated in comments above. Women do read tech news sites but I think HN design is more appealing to men. Not the core reason for the audience stat but an aside.