Fwelin 
"[@Background Pony #C5E2":](/1151468#comment_5018629
)
Because Starlight was completely repentant, apologetic, and appreciative of her chance to turn her life around ever since she was given it, while Trixie was an asshole as soon as she showed up again (in less than a minute, she mocked Twilight's title, made light of Twilight's forgiveness, and vandalized her property).
Attitude is a_*huge_* determiner on how lenient someone is willing to be with you. In this episode, Trixie acted like pre-Twilight's Kingdom Discord (at least, every time Twilight was around or involved); someone who is no longer evil, but is still a massive dick, and Twilight didn't like Discord during season 4, either.
Also note that Twilight never thought that Trixie was "evil" again, but merely that she wasn't a good choice for Starlight's first independant friend, and she was right.
Twilight's mistakes in this episode were her lying to Celestia and abandoning her with relative strangers in her castle, and trying to shove substitute friends in Starlight's face, not her suspicion of Trixie.
Because Starlight was completely repentant, apologetic, and appreciative of her chance to turn her life around ever since she was given it, while Trixie was an asshole as soon as she showed up again (in less than a minute, she mocked Twilight's title, made light of Twilight's forgiveness, and vandalized her property).
Attitude is a
Also note that Twilight never thought that Trixie was "evil" again, but merely that she wasn't a good choice for Starlight's first independant friend, and she was right.
Twilight's mistakes in this episode were her lying to Celestia and abandoning her with relative strangers in her castle, and trying to shove substitute friends in Starlight's face, not her suspicion of Trixie.