Probably because of hardware limitations, seems smash 3ds really squeezed everything out of the 3ds so you will not be able to access miiverse or post captures.... however the new 3ds is powerful and fast enough to allow this (more likely cause of increased RAM as well), here is a translation from Japan 3ds Smash Miiverse:
for can not start the Miiverse play in this software, you will not be able to post to the community screen photo. ※ If you are using this software in the New Nintendo 3DS / New Nintendo 3DS LL, offers Miiverse even during play.
https://miiverse.nin...558073037273117
SMASH 3DS is enhanced for New 3ds because of integrated support with new controls, and miiverse access as hardware allows.
I don't mind, I do think the devs used every resource possible for this game to run at 60 fps with 4 players and i rather have that then risking performance over miiverse during gameplay.
Edited by Sorceror12, 12 September 2014 - 07:26 AM.