Last I heard (go check the atlus USA forums for any updates on this BTW) is that Atlus dosent think the USA GBA market is really big enough to support the cost of localising and releasing many more GBA games, so they are kinda reluctant to do it.
From what I gather, their latest GBA releases (Summon Knight 2, Super Robot Taizen 1+2 and Yggdra Union) didnt really set the sales charts alight, so unless they eventually sell large numbers (you never know, the games could be 'slow and steady burners') they wont continue to support the format.
Shame I know, but judging by their future announced line up, Atlus havent stopped releasing quirky Japanese RPG's at all. Just not on GBA.