We suppose this makes sense given that the Wii U is a more powerful piece of hardware that could support that many players, not to mention playing it on a small screen like the 3DS could prove to be pretty troublesome. This was confirmed in a Nintendo Direct Video in which the company revealed that the 8-player mode would be a special mode.
“In a major first for the franchise, a special mode lets eight players fight simultaneously in local multiplayer. This option appears only in the Wii U version, and lets players compete on even larger stages to accommodate all the characters.” Now before you get too excited, Nintendo also mentioned that the feature would only be available on certain stages, and will not be available in online mode.
This means that there is a possibility that your favorite map might not be included for 8-player mode, and it also means you will need to bring 7 extra people to your home in order to take full advantage of it. It is unclear if this mode will be extended to online play in the future. The Wii U version of Super Smash Bros. has been pegged for a release towards the end of the year.