Elaborating a bit - brain is hard to study since you can't easily take a biopsy of it (from a living person at least), and various brain scans are not great at identifying the stuff we care about.

The slow acting nature of it means also you have to wait a long time to see results of clinical trials; also because early stages are easy to miss that also means you are stuck studying people who are already pretty senile and thus might be beyond the point where you can make a big difference.

Ruxandra has a nice piece, focused on cancer, but the reasoning is basically the same here: biology is just really hard. Sometimes we get lucky but in general it's a long, slow slog.

[1] https://www.writingruxandrabio.com/p/why-havent-biologists-c...