I always thought it was easier for men.To me, it seems most of the big girl YouTubers are beauty guru's. It just seems men dominate every other category, vloggers, gamers, sketches, animation, you name it.
Being attractive helps 90% of the time on getting a potential viewer, looks are first impressions after all. After the initial "Oh, this person is hot/attractive" the viewer will then decide if they like the content or not.
So when someone sees an attractive boy, they click on it and will watch their content. But if it's a girl, it's usually for a makeup video, so now the impression is "oh, she's a beauty person, moving on" because to the viewer, that's all that will come from her channel. Which in honesty, is the truth other than the occasional fashion/cooking/Q&A video.
I'm not saying all girls are makeup guru's, but considering YouTuber's fan-bases are usually girls, it's only natural they will be attracted to boys more than girls.
And to be honest, the only two female YouTubers I actually subscribed to are Superwomen and Communitychannel. Like someone else said, it usually all comes down to content. And these are the only girl YouTubers I found funny and liked their content for their content.