Its a hardware compatibility issue, the DS system it not capable of 3DS graphics. The graphics card in the DS plus cpu and memory was considered old even when the system launched in 2004
here is a technical comparison.
CPU two 266MHz ARM11
Memory MoSys 1T-SRAM at 64mb
GPU is Pica200 clocked at 133MHz
With 4Mb of Vid Ram
DS lite / DSXL
Memory 4 MB SRAM / 16 MB PSRAM
CPU 67 MHz ARM9 and 33 MHz ARM7 /133 MHz ARM9 and 33 MHz ARM7
the GPU is a Nintendo proprietary so I have no idea what it is exact but it is not very powerful.
At any rate as you can see there is a real physical reason, beyond that I read a article that tried to do it and the DS didn't even recognized something was inserted.