Previous version only made you click past the warning once. New version makes you click every time, and there's a noticeable delay before the config settings appear.
Previous version had comfortable spacing, with everything clearly readable without excessive wasted space. New version adds an absurd amount of padding to waste your screen space.
Previous version let you sort by column, so you could easily see all the setting you'd changed. New version removes this feature.
about:config is mostly used by the kind of person who complains about user-hostile policies and declines to accept advertising and spying, so if you want to do these kind of things then making it worse can help drive those users away. Simply deleting it is too obvious, but sabotaging the UX has plausible deniability. If you dumb everything down enough you'll only have dumb users, and dumb users are more easily exploited. I can think of no other reason why somebody put serious time and effort into actively making it worse.
(The old UI is actually still available at chrome://global/content/config.xhtml, but now that it's no longer supported it's easier for them to remove it later.)