The list of possible reasons are myriad.
The EVO fighting game tournament is planning its return to in-person competition this year, but Super Smash Bros will not join them.
EVO's Twitter posted a tweet today announcing the game reveal would air on their Twitch channel on March 8, but also announced in advance of the stream that Smash Bros would not be returning for 2022.
— EVO (@EVO) February 26, 2022
Smash Bros. games appeared at EVO in 2007 (Melee) and 2008 (Brawl) with a supported side tournament in 2009, and returned with Melee in 2013 after a successful charitable donation drive. Melee appeared in 2014 as well, and Smash Wii U joined it in 2015 - 2018, with Ultimate being the sole Smash game for the 2019 tournament. Ultimate was also among the games announced for the 2020 tournament before it was shifted to an online format due to the COVID-19 pandemic, with the online format being used last year as well.
In the two years since Smash Ultimate was announced for the EVO main stage, ownership of EVO passed to Sony following one of the main owners being accused of sexual misconduct, and Nintendo announced a deal for a major licensed Smash tournament circuit in 2022. Third party DLC characters "Minecraft Steve", Sephiroth, Kazuya Mishima, and Sora were all added to Ultimate since the planned 2020 lineup was announced.