Mainly because it doesn't produce an <html> tag. This error page is output when the database is down/unavailable. At least somewhat related to T149481, as there are also no lang or dir attributes either. Currently on REL1_28.
And while I'm here, can we maybe get an output hook in the outage HTML? I tried setting $wgExceptionHooks['DBConnectionError'], but did not get additional output. But this is an undocumented API, so maybe I'm doing it wrong?