The reason GBA had much higher sales in the US is because the US did not embrace the DS nearly as quickly as Japan and Europe did, so the GBA had a more normalized life cycle in the US.
But the GBA was not some odd outlier, the Game Boy Color released in November 1998, had similar yearly sales numbers to the GBA. Basically since the introduction of Pokemon and addition of a color Game Boy model in the US, sales of Nintendo handhelds in the US have been fairly consistent ... until this generation where we see a definite drop off due to the 3DS.
Nintendo handheld shipments (USA):
FY Ending March 2000 = 8.7 million Game Boys in the US (Game Boy Color)
FY Ending March 2001 = 7.74 million Game Boys in the US (Game Boy Color), a little drop-off here but to be expected because the Game Boy Advance had been announced and was on the verge of launch.
FY Ending March 2002 = 7.57 million GBAs + 1.46 Game Boy Color models (US market)
FY Ending March 2003 = 7.8 million GBAs
FY Ending March 2004 = 9.45 million GBAs (peak year, this is where the GBA SP model is introduced)
Nintendo DS launch, now lets take a look at sales globally here, and you'll see that the DS adoption is much slower in the US than Japan for example
FY Ending March 2005 = GBA = 2.34 (JPN) + 8.56 (US) + 4.49 (EU) = 15.39
DS = 950k (EU) + 2.12 (JPN) + 2.19 (US) = 5.26
We see the GBA still selling at a monstrous 8.56 in the US, which is its regular sales rate (more or less) of the Game Boy brand for the previous 5-6 years in the US.
(FY March 2006) - GBA = 1.0 (JPN) + 4.7 (US) + 2.6 (EU) = 8.3
DS = 4.78 (JPN) + 2.92 (US) + 3.76 (EU) = 11.46
19.76m total
Here we see again, weak-ish DS sales in the US, but still OK GBA sales.
FY March 2007 - GBA = 110k (JPN) + 3 (US) + 1.22 (EU) = 4.33
DS = 9.12 (JPN) + 6.63 (US) + 7.81 (EU) = 23.56
27.86m total
OK, so look at that difference, 3 million GBAs still be shipped to the US market, while only 110k to Japan. As of 2007 Nintendo is still kind of in the transition phase in the US market.
FY March 2008 - GBA = 220k (JPN) + 540k (US) + 820k (EU) = 1.58m
DS = 6.36 (JPN) + 10.65 (US) + 13.3 (EU) = 30.31
31.89m total
Now finally in 2008 we see the US market embrace the DS whole heartedly and the GBA is finally basically phased out to sub-1 million shipments.
The 3DS is just selling poorly compared to the GBC, GBA, and DS in the US market. There's just no shaking that. Nintendo has had handheld shipments/sales of 7-8+ million beginning with the Game Boy Color since 2000 in the US market alone.
It's not until the 3DS we start to see a baseline of sub-7 million numbers in the US market, to the point where we are today where Nintendo can barely ship 7 million 3DS' worldwide (this year)! This is not good, even the Game Boy Color before it was replaced by the GBA was selling 7+ million in the US market easily, now Nintendo can barely manage that with the 3DS in US + EU + Japan combined, and this is with a new hardware model (N3DS) in 2/3 markets this year. That's very poor by Nintendo's standards.