It would be easy for someone in the town to believe that Tituba works for the Devil. She's from Barbados, she's a slave, she's black, and she's a woman. Reverend Parris already suspects her; he said to Abigail at the very beginning that he'd seen Tituba "waving her arms over the fire" when he found the girls dancing in the woods, and there was "a screeching and gibberish coming from her mouth." Next, after Mr. Hale arrives, Mrs. Putnam tells him that "Mr. Parris's slave has knowledge of conjurin' [...]." The groundwork has already been laid for Abigail to accuse her. She is of very low standing in the community and two well-respected adults have confessed their reasons to be suspicious of her.
Therefore, when Hale begins to question Abigail about their activities in the forest, and she recognizes that she is losing credibility, it is too easy to name Tituba as the source of the problem. Abigail seems to have understood instantly that she could deflect any blame onto Tituba, and as soon as the slave is brought into the room, Abby shouts, "She made me do it! She made Betty do it!" It is as easy as pointing a finger. Then Tituba, fearing for her life, tells Hale and Parris what she thinks they want to hear: that the Devil did indeed enlist her to work for him and that she saw numerous others with him as well.