Because digit numbers are not foreign to Chinese. As simple as that. The majority of Chinese don't speak any western languages. A domain name of letters is no different from a random combination of letters to them, even if the name is composed of real words in English, Spanish, or French, etc. They are completely foreign. While a combination of digits is not, like phone numbers, zip codes, bus numbers, train numbers,…
I would be lost if you asked me to type in a Cyrillic URL (and Cyrillic is quite similar to Latin script, all things considered!), I wouldn’t even know where to start if you asked me to type something from an even more different writing system in.
I suppose English is on its way to gain some of the same universality as Arabic numbers, but it’s still a far way away from that.