Safari doesn't detect font installed on system
I believe that the reason for its being shown only in Chrome is most likely due to Safari's anti-fingerprinting techniques, which obfuscates currently installed system fonts from websites. However, as a by-product this prevents websites from displaying webpages from using a custom font that the user installed on the system (source).
Bear in mind that websites can still display custom fonts as long as they are loaded from a source external to your local filesystem, which is why a website like Google Fonts can still function.
Note: I am unsure why it's not possible for Safari to simply display the Roboto after the website has loaded. Surely that wouldn't reveal to the webpage host that Roboto was used, and thus installed on the system?