Because Snow White always "seeks the best in others" and lives her life trusting in that. Though it took the whole dang movie for this to happen----Rapunzel finally saw that Gothel was manipulating her. Ariel saw "the worst" in her own father and in Ursula, and part of her is more pessimistic than Snow White and Rapunzel are. It took the major destruction of her grotto, to prompt Ariel to do something entirely out of her norm--- which was make a bargain with the wrong person.

I hate when people say Rapunzel was easy to manipulate. I mean, for her Gothel was her MOTHER; so it is understandable that she trusted in her (she even lies to her own mother when she is planning to go outside).

We saw how Rapunzel didint believe Flynn`s lies at the beggining

She's the most trusting of others. She trusted Eugene when he took the tiara and said that there was something he had to take care of.