It's just copied off basic mailing list designs from the 90s (or the 80s?).
That design is basically what happens when you tell a kernel dev to write a mailing list archiving UI. It's clean because they value simplicity, but they obviously have ~0 non-CLI UI/UX experience.
lkml.org was ditched some time ago by kernel devs. It had often issues, is cropping messages, has no export function, has sometimes even ads on the site.
That's why kernel.org folks run now their own reliable archive.