The genesis wasn't popular in Japan either, but that never stopped it from being a popular console around the world, to the point that Capcom and Konami stated to make games for it as soon as they could.
Double Dragon was massively popular in North America, which has always been the largest market. It even made a big deal about it's arcade statues on the NES box:
http://gamefaqs.akamaized.net/box/0/8/1/3081_front.jpg
You didn't start seeing other games in the genre, as it was redefined by Renegade, by other companies, until after Double Dragon's success. Without that, who's to say Capcom would have even tried to make Final Fight as a beat 'em up, instead of taking another crack at the fighting genre, before Street Fighter II?