I mean Sunshine + Galaxy + Galaxy 2 + 3D World don't even add up to the sales of New Super Mario Bros. Wii

Super Mario 64 was more about "oh wow 3D" but games like Donkey King went back to 2D. Nintendo have said they are now focusing on the 2D structures of Mario.
And Super Mario DS was about nostalgia.

Why does 3D Mario flop compared to the 2D games? What's Nintendo doing wrong?

Classic. If it ain't broke don't fix it.

Idk, but the first Galaxy and 64 are some of the greatest games ever.

64 is incredible
Galaxy is just it borrowed elements from nearly every platformer imaginable and it's gets instant acclaim because it switches them to Mario sprites. They are solid games, but completely unoriginal and unexciting. I got Ratchet and Clank, PacMan In Time, Star Fox etc vibes all over.

They're easier to play for all ages. The 3D games are always better though imo.

The 2D ones are what Mario is known for, even though later editions have been boring and uninspired.

The 3D games >

Difficulty. 2D Mario is more universal and easier to grasp.

Growing up playing Super Mario Bros (The Lost Levels) I would have to disagree 👾

I finish each Mario 3D game Sunshine onwards in roughly 5 - 8 hours.

Games like Ratchet and Clank take me roughly 20 hours. (Except Nexus which I finished in 5 hours, and Quest for Booty which I finished in 3).

I don't think the 3D games were hard at all. I was 8 when Sunshine came out, 13 when Galaxy was out.

Nostalgia

I was around the same age when Sunshine and never finished it I still haven't actually (but I haven't tried in forever).

2D is easier to understand. + supposedly in Japan teenagers think 3D Mario is too kiddy.

3D >>>>> 2D.

I did everything lmao.
It's funny because people say that 64 was way easier, but playing it on DS (would have been 10) took me the longest

I never finished 64 (got all stars) and y'all finish the newer games in less than a day?

Those games are very successful.

None flops.

You also have to consider the console it is on.