Probably the same reason why most men don't read romance novels (91% of that readership is female). Or the reverse reason. Basically it's a matter of interests, and I would blame the difference in interests to nurture. It's hard, for example, to find women interested in World War 2 discussion (OOBs, which tank was superior to other, would there be a way for Sealion to work, who was the best general, Patton or Montgomery, etc). Since their interests don't go there, it's hard to expect them to buy a game about that.
I don't think Paradox games are sexist. They're one of the least sexist in their presentations. The EUIV loading screens show both men and women, and the women aren't shown to be inferior or sexual. It's true that most of the cover boxes and ads for the games show men though (conquistadors for EUIV, with some native americans here and there). Victoria's covers, even though the game is named after a woman, usually show Bismarck instead of her. The expansion show either Abraham Lincoln or a boat in the middle of a jungle. HOI's covers usually just show maps, miniatures, planes and sometimes an iron cross. Very neutral. Crusader Kings only show men, which is not so neutral, but is due to history.
Regarding how the game plays, 70% of the time is spent staring at the map and clicking, 20% staring at menus considering your options, 5% staring at the ledgers and 5% at the battle's representations to see what's happening and if you need to interfere with more reinforcements. You move units, and most of the time these units are represented by men, but these are historical games and war in this period was fought mainly by men. To make woman soldier sprites would be weird in this case. In HoI there's not even sprites, just NATO counters. Most of the pics for rulers, advisors, commanders or politicians are men, but again, history is the reason this is so. In CK2 there are more women characters, but most of the time they don't hold power and you don't have to interact with them, but that will depend on the succession rules of your realm. Only in CK2 you're actually supposed to have some interest in your character, he or she being your avatar. In the other games what matters is how many stars the general has or how much bonus the advisor gives or the stats for your ruler. You don't identify with anything, besides a country. So I don't think there would be a reason why a female player would play and then say "this game isn't targeted for me", as would be the case in games in which you're supposed to pick an avatar and there are no female avatars or only sexy female avatars scantily-clad.
Regarding how the community treats women players, Paradox's games are mostly played in Single Player mode. I never played multiplayer, and most people also probably didn't. I wish I could play HoI3 multiplayer (the AI is too dumb), but it's not strictly necessary for enjoyment. Also, a very small percentage of players actually post in forums or in the community page on Steam. In other games, where people talk with each other as they play, I can see how male players can make female players uncomfortable. But in Paradox's games? You're supposed most of the time to play against the AI. And the AI will not make uncomfortable comments.
TLDR: it's a matter of interests. There's nothing in Paradox games that makes them offensive to women. If women don't play, it's because they don't know how awesome the games are. You should do your part and try to convince a woman to play these games, because they don't know the fun they're missing. This could potentially increase Paradox's sales, which would result in more products, so it'd be fun to all.
I don't think Paradox games are sexist. They're one of the least sexist in their presentations. The EUIV loading screens show both men and women, and the women aren't shown to be inferior or sexual. It's true that most of the cover boxes and ads for the games show men though (conquistadors for EUIV, with some native americans here and there). Victoria's covers, even though the game is named after a woman, usually show Bismarck instead of her. The expansion show either Abraham Lincoln or a boat in the middle of a jungle. HOI's covers usually just show maps, miniatures, planes and sometimes an iron cross. Very neutral. Crusader Kings only show men, which is not so neutral, but is due to history.
Regarding how the game plays, 70% of the time is spent staring at the map and clicking, 20% staring at menus considering your options, 5% staring at the ledgers and 5% at the battle's representations to see what's happening and if you need to interfere with more reinforcements. You move units, and most of the time these units are represented by men, but these are historical games and war in this period was fought mainly by men. To make woman soldier sprites would be weird in this case. In HoI there's not even sprites, just NATO counters. Most of the pics for rulers, advisors, commanders or politicians are men, but again, history is the reason this is so. In CK2 there are more women characters, but most of the time they don't hold power and you don't have to interact with them, but that will depend on the succession rules of your realm. Only in CK2 you're actually supposed to have some interest in your character, he or she being your avatar. In the other games what matters is how many stars the general has or how much bonus the advisor gives or the stats for your ruler. You don't identify with anything, besides a country. So I don't think there would be a reason why a female player would play and then say "this game isn't targeted for me", as would be the case in games in which you're supposed to pick an avatar and there are no female avatars or only sexy female avatars scantily-clad.
Regarding how the community treats women players, Paradox's games are mostly played in Single Player mode. I never played multiplayer, and most people also probably didn't. I wish I could play HoI3 multiplayer (the AI is too dumb), but it's not strictly necessary for enjoyment. Also, a very small percentage of players actually post in forums or in the community page on Steam. In other games, where people talk with each other as they play, I can see how male players can make female players uncomfortable. But in Paradox's games? You're supposed most of the time to play against the AI. And the AI will not make uncomfortable comments.
TLDR: it's a matter of interests. There's nothing in Paradox games that makes them offensive to women. If women don't play, it's because they don't know how awesome the games are. You should do your part and try to convince a woman to play these games, because they don't know the fun they're missing. This could potentially increase Paradox's sales, which would result in more products, so it'd be fun to all.