I think it's due to the huge hype around chrome among developers back then. Each tab as separate process was huge. It also has built-in dev tool, and v8 being actually fast excites a lot of devs and drove them away from firefox. So when nodejs was released, a lot of people already know what v8 is and have positive impression of it, probably enough to nudge them to try nodejs.

On the other hand, I never heard of Rhino until now.