I don't exactly understand why people count pikmin as a main Nintendo franchise...

...but to your OP specifically: as small as metroid is it still has five games that have outsold pikmin's top seller.

i don't see pikmin finding a popularity surge due to the wii U.

It's because it's a Miyamoto game, and one of the only fairly large new  Nintendo games from the last 10 or so years to spawn a sequel.

Well I actually think that both Pikmin and metroid will sell better than usual. The Pikmin and metroid attractions in Nintendo Land should go a long way in advertising them and bringing in new fans. I think it's possible for both games to break 3 million, especially if Retro is develometroid

that's what, your explination for why pikmin will suddenly surge in popularity on the wiiU?

No, I think it will do better then previous Pikmin due to pre Wii U launch hype making it a  focus of the system to the fan boys, and also due to nothing else notable for the system coming out for a while.

I was answering your question:  why people seem to count it as a main Nintendo franchise, when it's always been a niche title.

if only Nintendo can fix their online and make Metroid like Halo for the WiiU.

oh, okay.  i wasn't sure before.

If Metroid is coming back quickly, i doubt it will be the Metroid people remember.

And yes, i would say Pikmin will outsell even if it comes.

I don't think many consider Pikmin a main franchise for Nintendo...yet.

Pikmin only has 2 games and despite it's 1st game being released over a decade ago, it is still relatively new and unknown to a lot of people. At least it was unknown. Of course Metroid will have higher selling games right now. Again, I think the problem Pikmin faced was no one really knew what it was. Since Pikmin 2, Pikmin has gotten a lot more recognition without a new game. It has been pretty hyped and fans have been waiting since 2008 when Miyamoto accidentley hinted at it. Olimar was in Brawl, the NPC versions were released, and Pikmin is featured in Nintendo Land. Pikmin also launched on the GC which obviously was Nintendo's weakest System in terms of sales. 

Overall, I think Pikmin stands a much better chance at being successful now than before. Pikmin 3 could possibly make Pikmin a bigger franchise than Zelda in Japan.

looking at recent performance pikmin is already the same size-ish to zelda in japan.

0.56 and 0.56 for pikmin 1 and 2 vs. 0.6 TP, 0.34 SS, 0.95 PH, 0.74 ST.