For RER2 to have been successful on 3DS it would have needed to be exclusive at launch. Even then, players might have simply waited for the inevitable PS3/4/X360/One/PC port.
I don't think sales in Japan matter much: Capcom was clearly aiming for Western markets with this sequel. I believe that's something that's already been said several times in RER2 discussions.
Unrelated: mixi announced that the Monster Strike anime has over 1.4m views on YouTube. Not sure when they wrote that statement, because it's way past 1.4m views so far (though most of views are from Japan). Unfortunately, it's hard to draw a comparison with other anime airing on TV since there isn't concrete numbers for those.