Please don't double-post, people get annoyed when you do that. Use the edit button instead.
Anyway, the other games were rereleased a few years back.
Final Fantasy Origins - I and II, Playstation
Final Fantasy Anthology - V and VI, Playstation
Final Fantasy Chronicles - IV and Chrono Trigger, Playstation
They are available at used game stores. I haven't seen any in major retailers like Wal-Mart for some time now, but stores like Electronics Boutique and Game Stop will have a copy or two laying around if you're lucky.
The original Playstation is dirt cheap nowadays, if you can find one. Last I checked they were going for some insanely low price. I have a PS2 though so I haven't had a need to pay attention to the prices.
As for why they haven't rereleased more old school Final Fantasy games on GBA...well, you'd have to ask the company. You could probably check any gaming news sites to find out if they have future plans.
~DragonHeart~