Tituba does not want to name others as witches. When she does so, it is because she sees no other way to save herself. Tituba tells them she does not work with the Devil, but they do not believe her. Unless she confesses, she will die.

Tituba: I have no power on this child, sir.
Hale: You most certainly do, and you will free her from it now! When did you compact with the Devil?
Tituba: I don’t compact with no Devil!
Parris: You will confess yourself or I will take you out and whip you to your death, Tituba!
Putnam: This woman must be hanged! She must be taken and hanged!
Tituba, terrified, falls to her knees: No, no, don’t hang Tituba! I tell him I don’t desire to work for him, sir.

She realizes denying seeing the Devil does her no good, so she says she saw him with others. When they ask who she saw, her first response is "I don’t know, sir, but the Devil got him numerous witches." They keep pressing her and ask her leading questions. This means they basically give her the answers, telling her what they want to hear.

Parris: Who came with him?
Putnam: Sarah Good? Did you ever see Sarah Good with him? Or Osburn?
Parris: Was it man or woman came with him?
Tituba: Man or woman. Was—was woman.
Parris: What woman? A woman, you said. What woman?
Tituba: It was black dark, and I—
Parris: You could see him, why could you not see her?
Tituba: Well, they was always talking; they was always runnin’ round and carryin’ on—

We can see that Putnam specifically asks about Good and Osburn.

Tituba's lines demonstrate that she does not want to name someone else. She tries to say she couldn't see who it was, but the men are relentless in their questioning. They will not rest until they receive a name. All of her answers are simply what they have already said; they ask if it was a man or a woman, so she responds "woman." Hale asks "Who came to you with the Devil? Two? Three? Four? How many?" So Tituba tells him "There was four."

Despite saying there were four of them, Tituba only gives two names: Good and Osburn. As noted, these are the names that Putnam has already suggested. Tituba doesn't pull these names out of nowhere. In order to save her own life, Tituba tells them what they want to hear.

In Act One, Abigail Williams claims that Tituba was able to conjure Ruth Putnam's dead sisters. Shortly after, Tituba admits to speaking with the Devil after being threatened by Reverend Parris and Mr. Putnam. Tituba then tells Reverend Hale that she believes that somebody else has bewitched the children. Mr. Putnam then suggests to Tituba that Sarah Good and Sarah Osburn were working with the Devil. This is significant because it provides Tituba with two people to cast the blame. Sarah Good and Goody Osburn both have terrible reputations throughout Salem and are viewed with contempt by the community members. Tituba, sensing the dangerous situation she is in, accuses both Sarah Good and Goody Osburn of witchcraft. Sarah Good is a mentally unstable homeless woman, and the two women are both considered easy targets. 

Tituba is in a truly pitiable situation: accused of witchcraft by Abigail, she is threatened with death by both her master, the Reverend Parris, as well as Mr. Thomas Putnam.  However, Mr. Hale is kind to her, and gentle; he asks, "You would be a good Christian woman, would you not, Tituba? [....] And you love these little children?"  He gives her an opportunity to speak for herself, and he doesn't threaten her with pain or death ever.  He asks her questions gently, and he believes her answers.  

Then, "Hale takes her hand.  She is surprised."  Tituba is shocked to be treated kindly because she is so unused to it.  Hale says that they will bless Tituba, that she is "God's instrument put in [their] hands to discover the Devil's agents among [them]."  He tells her that she is special and that she has a holy purpose, not that he will beat her or hang her as the others have.  Tituba seems to want to please Hale, to give him the information he desires: and so she names Goodwife Good and Goodwife Osburn as witches that she has seen with the Devil.  Once she does so, however, Abigail seems to realize the great power that is conferred on those who accuse others (after all, she accused Tituba and was believed, and now Tituba has accused two others and is believed), and she begins to make further accusations herself (along with her cousin, Betty).  

Tituba is led to accuse Goody Good and Sarah Osburn. She tells Parris and Hale that she had had dealings with the devil: "...And then he come one stormy night to me and he say, "Look! I have *white* people belong to me." And I look -- and there was Goody Good...And Goody Osburn."