Kynan Pearson(Former Retro Studios Dev, Currently 343) Interview

[curl-6]

Kaizar said:

Well, Mario & Sonic at the 2012 London Olympic Games, both versions started development at the sametime, but the 3DS version toke 4 or so months more to develop.

Most likely not due to graphics though, probably gameplay being redone for the touchscreen. 3DS has  a more standard graphics setup, easier to work with than Wii.