Because in a movie, an actor can use their charisma to sell any character, and they only have to do it for a short amount of time in ONE story.
In comics, characters need to stand out for many stories, in actions and written dialogue. In Suicide Squad, Tressor stood in contrast to the likes of Deadshot and even anti-heroes like Rick Flag. Eventually, his plotline climaxed with him threatening to beat the hell out of Waller, to her face.
Somehow, I doubt Diana gave him cause to make threats like that
Comic characters need contrast, and they need a hook. Tim is the family Spock, joining to balance out Batman's darkness. And a big part of his arc was that he was the least capable of the Batclan (at least when introduced)