This might be an unnecessary bump, but I find topics like this to be very interesting and on a personal note I kind of feel the need to air a few mild grievances I have had with some parts of the metal community which, much as we may be tnankful for it, has had its fair share of problems like any collection of people.
Just one thing I'd like to point out beforehand is that I think you have a slightly malformed question. When talking about subjects that are this complicated, as social subjects tend to be, you can't just look for one singular reason. It's likely to be a combination of several things that were mentioned ITT.
Biology by itself cannot explain the effects we have seen. Biologically, I don't think women have changed much in the past century, and yet if we take the last hundred years into account we will see enormous differences in the types of interests that women have expressed. There are more women getting into stem fields, getting into combat sports or even just sports in general, they have become a sizable demographic in the video game industry....the list goes on. As I said previously, in terms of biology women haven't changed much in the past century, so I don't think this aspect will contribute much if at all to the disparity. What has changed is the culture. We live in a post-civil rights era where individuality and self-determination are valued above erroneous moral principles that pretend to derive themselves from nature. Even still, after all of that, we're still trying to outgrow some outdated preconceptions of what a man or a woman should be. That said, I'm sure there may be some aspects that act as minor inhibitions - smaller fingers, less fast-twitch muscles, etc - but I don't think these would be enough to account for such a large disparity when we know the average metal musician isn't playing a 666 bpm blast beat on a quadruple bass pedal. Typically, metal isn't a competitive sport, lol.
When considering culture, you also need to take into account what culture has surrounded metal itself and the type of effect this may have. For a long time and even still today there is a great amount of negative stigma that surrounds the genre of metal. Everyone here knows that well. If you grew up around kids who were both religious and ignorant or you had adult figures who were equally closed off to new ideas, you certainly knew that stigma. "It's just noise", "you're summoning the devil", "smart kids shouldn't listen to stuff like this, they should listen to Mozart", on and on. So, already out of the gate even if you're a guy you were going to get comments like this if you had such environments. If you add on top of that the type of expectations that will be put on you as a girl, you are just completely fucked, lol.
These views aren't that prevalent any more, but at the time that Simone Simons was growing up it was way worse than when I was in high school getting crap over my proud ownership of a Death Magnetic CD - which, granted, anyone who stans that album unironically deserves to get bullied BUT STILL, it wouldnt have made a difference if it was DM or Lightning.
That said, this does segway into my next point over how cartoonishly toxic some parts of the metal fanbase itself tends to be at times......I understand that the whole general idea of being a social pariah has been effectively romanticized and is sort of an essential and (in my opinion) unfortunate hallmark for some metalheads, but this whole attitude of "you're not a real metalhead if you listen to x and y or you dont do z" can honestly be kind of ridiculous. These expectations make it especially hard if you're some poor newb who just wants to casually jam out to some A7x and tbh I think this is why metal can be completely inaccessible to most people. There are some beautiful acts in metal that are just master classes in everything from tonality to rhythmic complexity, but at times the fanbases of these acts can be too....fanbase-y. And that can really turn off potential metalheads from even listening to the crazier stuff when they're being told their tastes are wrong, let alone getting into it enough to want to become an artist in such a genre. Art is meant to be shared and enjoyed. If we want these ideas to survive beyond our times, we are going about it completely wrong.
All things considered, it's a wonder we have frontwomen like Angela Gloddow and Alissa White. There is such an insane amount of pressure that works against you, doubly so if you're a woman, that I do think should be examined because it does reach a point of legitimate concern at times. All of us are here, sure, and I'm sure none of us care what anyone says about our tastes or how we look or what have you, but not everyone is the same way or grew up in the same circumstances, and we won't all have the same priorities as a result. If we truly want to carry forward the same spirit of self-determination and encourage everyone to be who they are, then we should be more critical of attitudes that only serve as barriers.
One final point before I end this long rambling post: about your observation over more frontwomen as opposed to instrumentalist, my hypothesis would be that this is a feedback loop effect. To put it simply:
new girl sees metal frontwoman -> girl grows and becomes a frontwoman -> new girl sees metal frontwoman
In a highly selective and risky business like music - especially metal where female role models are even rarer - I think this is a likely explanation. Girls grew up seeing women singing and fronting an otherwise all-male band. This has predated even the existence of metal itself. Thus this can pretty strongly influence the kind of image they will have for themselves to pursue, and in turn will influence their fans once they grow up and fulfill their own dreams.
hope this was an insightful post, and hope i dont get banned for necro if this counts as that. sorry if it does, but Im just very interested in this subject and wanted to put my thoughts here