From what I understand it's simply the fact that the DSTT doesn't have the hardware required to run GBA games. That's why you can only do it with 1) the additional Slot-2 expansion or 2) a DSTWO flash cart, which contains the required hardware itself. The reason that the DS can run GBA carts is because the required hardware is within the GBA cart itself, but it leads many people (including me) to believe that a DS should be able to run GBA ROMs.
I was just wondering if someone may have found some sort of workaround to this problem, but it appears not.

Ah well, not too much of a problem. At least I can still play GB/GBC games with Lameboy