They only recently found out how those 512Mbit games worked, as far as I understand the GBA can only address 256Mbit. So for the 512Mhit games to work rk on the GBA, Nintendo needed to do bank switching.
I doubt anyone is going to invest time and effort into implementing this in a flashcard when there are just 2 or 3 titles that use it and those titles are movies in what can be considered crappy quality.
In fact, we did some research on the video rom, they need to map the memory bank twice. Later, we found that it is not worth it. After all, who will see such low resolution contents on the small screen now.