I think it has to do with the decade-long and unique Franco-Belgian tradition with BD (bandes dessinées). [I'm from Germany and love BD way more than manga, btw]. Starting with Katsuhiro Otomo's Akira in the 1980s, Japanese manga publishers expanded their audience to Europe and NA. Akira was a sensation among western comic fans. As fas as I know France embraced manga first and faster than other countries (and imo thanks to their already established BD culture). I also think that early Studio Ghibli movies were most successful in France outside of Japan.
[Side note: The downside of today's worldwide manga domination, at least for Germany: Franco-Belgian BD were pretty successful over here 20 years ago. Today people only buy the latest Asterix (1+ million copies in Germany alone), all other BD - even those who are successful in France, like, say, the latest Largo Winch or the latest series from Scotch Arleston - sell only several 1000 copies in Germany these days. In other words: While France had adopted manga earlier and still have a very successful BD market, the German comic market and presumably all other markets in Europe are almost completely dominated by manga today.]
Also, BD and manga influenced each other, e.g. when I first saw the Ghibli movie Nausicaä (I think it's originally from 1984, but made it to the west only in the 1990s), it was pretty clear to me (I'm a Moebius fan) already after a few minutes in - when Nausicaä discovers the Omu (sp?) shell - that Miyazaki was strongly influenced by legendary French BD artist Moebius' (real name: Jean Giraud) works from the 1970s for Metal Hurlant: Arzach and Le Garage hérmetique (these comics were groundbreaking and highly influential in Japan). For a Moebius fan, the influence on Miyazaki is almost instantly visible in his early movies. For all others, here's an article on the topic for further reading: http://www.nausicaa.net/miyazaki/interviews/miyazaki_moebious.html
So it's no wonder to me that France (and I assume also Belgium) embraced manga, anime and early Japanese videogames such as Nintendo's faster than other countries. FRA and BEL (to a certain extent also NED) are the only countries besides Japan with an own and unique comic culture which is completely different from the US tradition of superhero comics.
