I think because it's still kind of a taboo to write a woman with flaws in fiction. It's much more normal to write flawed dudes. Either we get over them or they make us villans. I am not a fan 2 Broke Girls, hotness of the two leads aside, but from what I've seen, as sitcom-y as they are, they have distinct personas and are flawed characters.
I wouldn't say them not being flawed is the issue. By someone being a "weirdo" I mean more that they violate social norms of how a "normal person" should act. Sometimes that happens because of a person's flaws (I keep going back to Batman and Daredevil), but it's not always true. Characters Tony Stark and Peter Parker aren't "weirdos" because they're flawed, but have a distinct mode of behavior that we're told is contrary to how a "normal person" would behave (and they both don't care, which makes them interesting). Peter and Tony still violate a lot of those norms even when having their strengths emphasized.
Having more flawed female characters would add more diversity, sure, but that by itself still wouldn't fix the issue. I'm talking going even bigger. The other two brands of "weirdos" I brought up would still be largerly ignored as far as female leads go. There shouldn't be any limitations to how different from the normal crowd a female lead could be, just as there's currently no limitation on how different from the normal crowd the male leads are.
I guess my point is that a lot of people are all talk about female leads. They want to fix the diversity issue, but they're not willing to fully abandon the idea of a woman not being allowed to violate norms to the same extent as a man. It's a double standard against women that even the most well-intentioned writers seem to have, and that's the frustrating part.