I don't know where you get these numbers but they're wrong. Nintendo's last reported figures from their 1st half results:
http://www.nintendo.co.jp/n10/news/041125e.pdf
(last page)
shows "Life to date Sept. 2004" sales total for GBA (including SP) of 14.56m in Japan, 28.61m in The Americas and 14.96m listed in "other" i.e. rest of the world including Europe.
Now compare that to Sony's PS2 breakdown of sales by region:
http://www.scei.co.jp/corporate/data/bizdataps2_e.html
As of the same date, September 30th 2004 the sales are 17.89m in Japan, 30.23m in US and 25.88m in Europe. I think this validates my original point, room for expansion in the European portable market, not so in Japan and America.