I'm not sure exactly why it is to be honest. I've heard a few theories on why though.

Disclaimer: these are theories I have heard from people in the community, they are not my own.

1. There are just more transfem people on Reddit and reddit-like platforms due to there being more males on Reddit and thefore more transfem trans people:

One theory I've heard from people is that because Reddit is supposedly meant more for men than it is for women a larger majority of trans people there will be transfem. It seems like a plausible reason for why there are so many transfem posts but it does little to explain the hostile treatment of transmasc and enbies on its own.

2. There are more transfem people than transmasc + nonbinary:

Another theory I've heard is that there's just more transfem people than transmasc and nonbinary. This one doesn't seem likely because there isn't much evidence to back it up besides the abundance of transfem posts in the community and also people claiming it's better (that's very subjective by the way, transmascs would say the same thing about being a man). On top of being unsubstantiated it also reeks of terf rhetoric since I've heard similar reasoning from them i.e. trying to say men will go extinct because they'll all become women 🙄. I've also seen quite a few polls with numbers significantly higher in transmasc and nonbinary numbers so that kind of dismantles this one then and there.

3. Transfems are louder to combat more stronger transphobia

This theory basically says that it's not that there are more or less of any group but rather that transfems are louder in order to combat the immense amount of more direct and/or violent transphobia that they are targeted with. I heard this one in a trans community in a discussion about violence towards trans women. In some ways it makes sense since often trans women are more often seen as predators and attacked for it though it doesn't mean they experience more transphobia than anyone else. Transmascs still experience quite a lot of it but it's more subtle and typically less violent (except in cases where they're mistaken for trans women which sadly isn't uncommon). This one also reeks of terf rhetoric to me since I've heard lots of them say that transfems are loud and proud so I can't say for certain how accurate it actually is.

These are just a few theories I've heard people talk about when it comes to this phenomenon, there are also a few others but I'm not going to bring them up because they're either straight up completely bigoted arguments or they don't make any coherent sense whatsoever (sometimes both). Also it's important to take theories like this with a grain of salt, they may have elements that are true to them but they also have issues with them as well which could lead you astray. I don't think this isn't an issue it definitely is and I know a lot of transmascs and nonbinaries who have experienced invalidation in some way related to this or have been told that we matter less. I'm not really sure how we could solve this problem though.