Here's why:
* Ultimate Alliance 2 was released in 2009. Deadpool's popularity exploded after that, becoming a major player in the Marvel Cinematic Universe in 2016.
* The game's focus was on a different narrative. Ultimate Alliance 2 focused on the Civil War storyline, featuring heroes and villains aligned with Iron Man and Captain America.
* Deadpool's character was less prominent in that era. Though he was a popular anti-hero, he wasn't as widely known or as mainstream as he is today.
So, while it's possible that there could have been plans to include Deadpool, ultimately, he wasn't featured in the final game.