Also, she's relatively progressive so I don't see why she's such an anathema to Forum Democrats - except for the fact that she's female, but that's all I can think of.
Policy-wise, DiFi is more hawkish than the voters she represents and takes a fairly authoritarian view of civil liberties (see e.g., her stout defense of NSA domestic surveillance and her frequent tiffs with Sen. Wyden). There is a significant portion of her voting base for which those positions are unpopular. Roll those in with rejecting single payer and there's a non-gender reason.
Although there may be a portion who are anti-DiFi due to her gender and support of LGBTQ issues.