@ianpony98  
But then those without a WiiU, or whom got the 3DS version instead (seeing how the WiiU console was nowhere near as popular, nor sold anywhere near as much as the 3DS) would probably complain about that exclusive.  
That’s probably why both versions have the same characters. (But different stages as some would better suit the smaller screen.)
 
I’m curious if SSB for Switch (I bet they’ll make one some day) will have any new characters, or stages like the Splatoon Inklings. Maybe they’ll bring back the Climbers in it. (I’d like to see an Ice Climber stage done more like the 8-bit original game, with breakable blocks, moving platforms, Topis, flying birds, and such.)

@Beau Skunky  
I sure wish they were Wii U exclusives meaning you could only play as them on the Wii U version

@ianpony98  
Shame, but apparently it was due to getting them to work on 3DS. (16 Climbers especially in online mode would be very very laggy.)  
They could’ve split them into one character to make ‘em work better, (and it’d be interesting to see Popo & Nana play separately) but then I guess they would lose their “playing as 2 characters” gimmick that made them more unique. (Though, Duck Hunt duo, and Rosalina & Luma have that gimmick too.)
 
Plus, now we have tons of characters, with fire & electrical attacks, but no ice ones. (Aside from Lucus’ PK Freeze, and Kirby’s unlockable B-button ice custom.)

@Beau Skunky  
What dissapointed me the most was when Ice Climbers was cut from Smash Bros for 3DS and WiiU

I’m actually disapointed they never made a WiiWare or something Ice Climer remake. (Due to SSB making them a cult-classic.)