Because nobody except for HN commenters cares what lamguage it's written in. It's about as easy to use and flexible as it gets so development is fast and people can contribute easily and it's entirely fast enough if you run it on reasonable hardware (for proof see other comments around here).
The fact that it doesn't run on your toaster is probably a feature, not a bug. To get it running on a Pi, even if written in a compiled language, things would have to be stripped down, making the overall software worse - a for what reason?
Running a Matrix server on a Pi in your closet is about the worst thing you can do. It's slow, unstable, has zero redundancy, needs constant maintenance, is physically in your home, on a residential connection.... If the point were for users to run this themselves, something like a local NAS (Synology has 1-click apps, I believe) would be the lowest-end reasonable option, but the whole idea of running your own Matrix server at home is insane! At the very least, it should have basic redundancy for uplink, data and power, which are already near-impossible to achieve at home.
And why should servers hosted by individuals? That would be horribly wasteful and inefficient, as well as raising the bar for users. I'll guarantee you that anyone who has enough skill and experience running and maintaining servers will have more than just a Pi available to them and anyone who doesn't have that experience should really not be running such an important service themselves.