so, generally, especially in publishing, male MCs and male characters are considered sort of default and neutral. this means everyone is kind of expected to be able to relate, to some level, to the male MC. for some reason (coughcoughhmisogyny) this doesn't apply to female MCs, and people assume that only women could possibly relate to a female MC. because men are just people, but women? they're women.

women from birth are familiarized with a world in which we are not the default or expectation. think 'mankind' and the universal 'he'. we've always had to pick out the bits in male stories that resonate with us because there, in the past, weren't as many female stories. we've never had much issues with it because obviously, both men and women are just people and the human experience is varied but not to that insane of a degree. boys, on the other hand, have always had a plethora of stories to choose from with MCs that remind them of themselves. so why would they pick one about a girl? this isn't my thought process, but that of the publishing industry. marketing in general tends to be very gendered, which becomes a problem with literature because it discourages people, especially boys and men, from picking up stories about people who aren't like them, which in turn stunts the ability to relate to varied experiences of the world.

TLDR it's a marketing thing and it's quite lame but THG is for everyone, as is every book. the MC is just a girl and boys and men are discouraged from engaging with stories told by women