with luma ntr hans etc you can redirect the rom of the game from the sd card for mods but in ds mode you can't run retial games off the sd yet except for demos and maybe dsiware roms

 

Several reasons:

1. 3DS roms are intended to be run from multiple locations. Cartridge or eShop, and possibly Virtual Console in the future. So, they call system APIs, which we can replace. DS roms though don't have system APIs, they interact directly with bare hardware to read the cartridge slot.
2. When running in DS mode, the SD card is turned off, and we don't yet know how to turn it back on. (not the case for DSiWare)

 

ncch's were designed to be versatile while srls weren't.
You can run an ncch from different media by just flipping a few flags while and srl has to be recompiled to do the same thing.

that's a simple non-technical answer. no reason to go into any more detail so don't ask.

 

thing is do ds games have a rom file system like 3ds games

 

They have a .app but its not compiled the same way as a .nds, its really different iirc

 

nds expects (at least commercial game) data to be pushed to and read from slot1. That's basically all the more there is to it. The problem is effectively emulating the card read from within nds mode (or somehow redirecting it to a device that the system isn't expecting to be there, which from my understanding they've made some really good progress in this area).