Thanks a lot to you all for your replies !
They are very useful for my research.
Actually Ben and jerry's tried but didn't succeed because they had distribution issues and it was hard for them to compete with already established brands... What's more I think japan industrial structure of keiretsu is complicated and make it difficult for foreign firms to come to Japan.
So they were very briefly available at 7/11 stores but no one noticed apparently !
Is that true that Japanese people are very demanding in term of quality of the items they purchase ?