Coded was definitely easier to do overall, but with Days it was probably because the scale would be huge. We had models and everything for most things so the graphical department would've been done easily for the ps2 era, but reworking the multiplayer would've been pretty chaotic. Specially since it would've been featured on future ports and stuff.
It was simpler on the DS, but having the game on a console-scale meant they would have had to pay much more attention and update more than just the basic gameplay, as well as caring about online servers, so I feel like that was a bit too much for their goal at the time.