It's probably because ultimate doesn't really feel like a new title, more like just an enhanced version of the WiiU release. 

It's more of a jump from Smash 4 to Ultimate than from Brawl to 4 content wise.

Smash Ultimate is simply releasing much earlier in Switch's lifetime than it did on 3DS/Wii U in Japan. (1 year and 9 months in on Switch vs 3 years and six months in on 3DS)

I know it's good to have opinions and stuff... but how can you still think that after the August direct? So much content that wasn't in the Wii U version. There's no doubt that Ultimate won't outdo the Wii u+ 3ds version in sales too

Reminder that Odyssey finished with less points than 3D Land, and had the highest 3D Mario First Week. However, Ultimate has the bundle which releases before the game, so it has that going against it. Tho idk how MC or Famitsu will count them...

hope kingdom hearts will get some points soon, because lately...

The combined numbers should pass ultimate in latest 5 days (if nothing unexpected happens).

It's just extra content on the same title. It's like Final Fantasy XV and Final Fantasy XV Royal Edition. Royal edition has tons of extra content, so much content that wasn't there in the original release. I'm not bagging SSBs, I'm picking the title up myself. But I don't feel like it's so different from the last title, especially after I bought all the dlc for SSB4 and it's now included at launch with this release. 

Since RDR2 isn't setting fire on the chart, do you guys think it will have GTA 5 legs in Japan?

Come on RDR2, you can do better than that!