Because Chromebooks are cheap, nobody spends money on them, so they’re perenially underpowered.
You've got that backwards. Chromebooks are supposed to be cheap, which means they're low power, which means they need a lightweight OS. That's what ChromeOS is for.
Makes sense to make a clear delineation for the market with different branding, hardware and OS.
This isn't branching into a different market. It's a replacement for ChromeOS.. Which means the ChromeOS market segment is just being abandoned entirely.