It would make it hard to spot impostor domains like "news.усомbiнаtor[.]сом" if it was. There's enough inertia for FQDNs to be strictly ASCII and any UTF-8(outside ASCII) in domain names to be felt unnatural for an URL, so most systems default to the raw "Punycode" xn-- scheme for all IDNs.
Safari at least shows the proper マリウス.com - I believe it has more complicated heuristics that boil down to "if it looks like a real script, show it, if it looks like some mangled English, show the xn-"
