I would guess that the establishment has to open a wallet on an exchange, in this case probably litebit, and set up their software for it. For whatever reason, they picked DGB, and now RADS. I just checked DASH vs DGB on coinmarketcap for the volume traded on that exchange and it's much higher for DGB. That might be part of the reason.
Why do they take DGB? Probably for the transaction speed. You don't want to wait around for 2 hours while your bitcoin payment is confirmed!
of course, i understand that, but I mean why DGB and not any other type of ALT coin
I would guess that the establishment has to open a wallet on an exchange, in this case probably litebit, and set up their software for it. For whatever reason, they picked DGB, and now RADS. I just checked DASH vs DGB on coinmarketcap for the volume traded on that exchange and it's much higher for DGB. That might be part of the reason.