The typical response might be because they included these sports in Wii Sports or that these games never sold that much on the N64/GCN.  However, I believe that these two series would have been pretty damn fun with the Wiimote and a lot more fun to play than the Wii Sports golf/tennis.  The N64 and GCN Mario Golfs were very fun to play (I even had a blast with Mario Golf on Game Boy Color).  Even though the GCN version made it easier to play (over the N64 version), it was still a blast to play.  I believe these two games could have sold at least 2-3 million each if done right (especially with the WM+).  Instead of relying solely on Wii Sports Resort to push WM+ they could have followed Wii Sports Resort with these two games to help push WM+.  The least they could have done was release a New Play Control of the GCN Mario Golf like they did with Mario Power Tennis (didn't like the GCN version as much as N64, played without the power moves most of the times on GCN version).

Mario Tennis (N64) - 2.32m

Mario Power Tennis (GCN) - 1.22m

New Play Control! Mario Power Tennis (Wii) - 1.4m

Mario Golf (N64) - 1.47m

Mario Golf: Toadstool Tour (GCN) - 1.53m

Why didn't they release a new Pikmin?
Why didn't they release the vitality sensor?
Why didn't they release more 3D games?
Why
Why
Why

EDIT: Soz, I'm a little negative towards them lately. Like Rol said in another thread, the Wii has the most untapped potential of any console this gen. Also (same source) they didn't really support the Wii enough.

A new Pikmin might be better suited for the WiiU.  It would work fine/great with the Wiimote but I believe the WiiU controller would make it a more fun experience.

I never really saw too much potential in the vitality sensor.  Sure, it would be neat for a fitness or horror game but besides that I see little use. 

I'd love to know that myself really. A NPC Mario Golf would have been an easy 1m+ seller. An all new Mario Golf probably double that. After NPC Mario Tennis I certainly didn't expect a new tennis game.

There were other titles that could/should have gotten the NPC (or all new entries) too: Luigi Mansion would have been perfectly suited to the wiimote for example. While they didn't have a GCN version, Pilot Wings and Pokemon Snap would also have been brilliant on Wii. But instead two of these have gone to 3DS which annoys me, especially when it'd be fairly easy to port them to Wii as well.

im just gonna go with laziness, sales my ass, not every game is going to do NSMB numbers, that crap that SE made went to do like 1.2 mil?? THey were not even trying, how much better could a good game made by Camelot would have done??

I agree with a NPC Luigi's Mansion.  At least it looks like Luigi's Mansion 2 will be a very solid title for the 3DS (some are saying the 3D effects are some of the best yet on the 3DS).  How did the NPC Mario Tennis game play/control?  I was more of a fan of the original Mario Tennis (N64) but the GCN version basically played the same (if the power moves were turned off).  The power moves were neat at first but I disabled them after awhile.  I am also curious about NPC Pikmin.  Using the Wiimote seems like a perfect setup for Pikmin series even though I said earlier in a reply to Dr. Grass that it would be a better suited game for WiiU controller (was thinking the controller screen could be used in a neat way for the series).

that is sooo true...I was waiting for mario golf to come out since the Wii's inception...
but they did not release the game...they did not release other titles either from their
huge catalog...

Wait wait wait... are you telling me that the atrocity of Wii version outsold the outstanding GC version?

You know it's true. There are so many possibilities left with Wii/DS that are going unanswered that Nintendo probably could have (and should have) continued to focus on them instead of 3DS/Wii-U.

@sethnintendo - Y'know, I have NPC Mario Tennis, but like many in my collection, I haven't really played it (I just like having a collection!)  From the little I played, the controls work well enough but I can't say it enhanced the gameplay any.  Probably did introduce it to a new audience though.

Re: NPC Pikmin - Did spend some time with that.  Not a lover of Pikmin or RTS in general but absolutely, the IR controls definitely suits that title and it feels like it was designed for Wii all along.

NPC Donkey Kong was also great bringing a great game to a larger audience to enjoy without the need for bongos but still giving the same tactile feel of the bongos.

makes no sense to me either, but to be fair by the time the NPC version came out it was really hard to find the original GC version. it was being sold for like 40 bucks used