because they appear to work on her target audience.
Charm is all a matter of choosing the right target audience. If you watch a video of any of Hitler's speeches he can look awkward, even downright goofy to a modern audience from a democratic country. And yet he was insanely popular at his times.
Charming someone is the art to make them think that you understand them, that you're on their side, and that you think they're well worth your attention. Different people react in different ways to different messages. A relaxed, mellow person won't be impressed by a rant, but someone with a chip on their shoulder might be.
Rebecca Watson has a huge appeal on the kind of people who are atheists but still want to feel morally superior to others, to improve society through their activism, and to punish those they consider to be immoral. She and PZ Myers charm people who feel wronged by society for one reason or another and dream of revenge.
Liberal arts students think that they deserve a good job because they achieved academic success. Society teaches you that if you study hard and get an advanced education then you'll succeed professionally. However this is only true if the skills you acquired through your studies are valued highly in the job market. Engineers, doctors and lawyers are paid well, and the latter two professions are seen as glamorous and highly attractive. Someone who does really well in medical school or law school is likely to get a good job.
Social or gender studies, on the other hand, are pretty much useless to find jobs, let alone good ones. This is true for almost any branch of liberal arts. So someone who has done well academically but still struggles to get a job feels cheated of a prize. The cultural settings in liberal arts degrees is predominantly leftist, so this kind of people already attribute most social problems to the Patriarchy.
Enter Myers, Watson and the SJWs. Their message is all about how society is flawed because old white males have too much power, and other groups are horribly oppressed and deprived of what is rightfully theirs. It's a message that contains some degrees of truth, but it's simplistic, brought to extremes and dogmatic. However it appeals to someone who feels bitter and struggles to get by. If you're a woman or a member of a minority who has been a good student but fails to get what they want, it's not your fault because you picked the wrong degree. It's the oppression of the Patriarchy.
If you're a white man who has been through the same process and feels the same way, you're not useless. You can be an ally in the fight against oppression. Sure they don't give you cookies but you still matter, you're still better than the rabble, your posts on the Internet are meaningful activism, and you can be proud of yourself to a certain degree.
It's a really powerful message for someone who already leans towards leftist positions and struggles with feeling of uselessness and inadequacy. Myers & co coddle their audience, give them an identity, a purpose in life. Of course activism needs an enemy, and here the "harassers", the "dudebros" come in handy. Anyone who isn't a devoted follower is an enemy, and has to be fought in any possible way.
The Horde is insulated by external influences by a liberal use of the banhammer. Hatred against the ideological enemies is justified and encouraged. Critical thinking and freedom of speech are derided and substituted by memes and by the "shut up and listen" approach. SJW dogmas are not to be questioned.
And so anyone who invents a witty insult against the dudebros, or rants about his woes, is celebrated, offered pouncehugs and affection. The aforementioned liberal arts student who struggles professionally becomes a hurting warrior, even a tragic hero.
Sure, there's always the risk of falling short and be cast aside because of your sinful patriarchal thoughts. But there's an easy solution for that: make more and more outrageously extremist claims, agree with everything that the leaders say, and use your Twitter weapons to fight the good fight.
