Because that tends to be how Nintendo operate on Switch. Personally I don't see last September as being disappointing; I think it's important to remember though there was a lot of chat in the lead up that didn't pan out (Metroid Prime and Zelda HD titles for example were widely expected despite the latter not making much sense) - plus while Nintendo did get the Tears of the Kingdom date out, they barely lifted the lid on it once again despite expectations there'd be more shared. I think those factors contributed a lot to the reaction last year.
February and sometimes June are more announcement heavy in terms of first party titles, while September is slightly on the lower end. 1 to 3 brand new reveals this month would be about par for the course and in line with the last few years, without being as high as this June which saw 5 game reveals plus the first gameplay for Detective Pikachu.
You'd have a pretty packed Direct:
- updates on the 4 titles coming before Christmas
- 1 to 3 new titles confirmed
- DLC detailed for an unknown number of games (Pokemon, Splatoon 3, Mario Kart? Smash? Zelda?)
- potentially updates on 1 to 3 previously announced games for 2024 (Peach, Luigi's Mansion 2, Metroid)
Unless Nintendo plan on dropping the February Direct, or revealing new hardware before the fiscal year closes out, there isn't all that much need for them to be particularly aggressive this month. They have a slate of games and DLC incoming and 2024 is beginning to shape up.