We don't have a clear reason for that, but it's probably because she is so distraught at what she now realizes she must do--go to court and confess that she and the other girls have been lying. When she does finally begin to tell her story to the court, Proctor must kind of "prop her up" because she's so frightened of what is about to happen. The confession turns into a mockery and a disaster, just as--I'm sure--she was afraid it might. It probably took her a week to get up enough nerve to face Abigail and the rest of the girls.
I'm not sure your statement is true. Mary does take center stage for a time, it's true; however, even in doing so she is inextricably connected to Abigail. She's afraid because of Abigail; she is tormented by Abigail; she recants her truth and reverts to a lie because of Abigail. That makes Mary less important, it seems to me, than...you guessed it. Abigail.
On the other hand, Mary is in the right, a position of truth--something Abigail has never come close to reaching. Mary's attempt at telling her truth in the face of such pressure is a foreshadowing of Proctor's dilemma at the end of the story. He, too, fails...until he recovers his faith and receives his forgiveness.
Mary's position as a spokesperson in the courtroom represents the kind of honor and regard someone of low station could attain when they became embroiled in the hysteria and mob reactiveness of the witch trials. Her previously unimportant social position gives way to her being someone whose opinions are not only listened to but become instrumental in determining the fate of the accused. Witchcraft was seen as the work of the devil, and punishing witches was seen as the good Christian thing to do.
Similarly, during the McCarthy era, which this play is meant to illuminate and comment upon, people who were called upon to implicate their friends, family and associates were made to feel that they were serving a higher purpose, because Communism was seen as a kind of evil force and harmful influence taking over the United States. It wasn't necessary to be of strong character (like Abigail) to be listened to; the meek Mary Warren represents those who were manipulated and seduced into testifying against their compatriots in exchange for a greater sense of importance and being part of a larger noble cause.
If there is a precise answer to your question, it would lie in the thoughts of Miller, himself. This would require an analysis and research that might prove to be more lengthy than the space provided here. In its place, we can only offer reasonable thoughts as to why this is the case. If your assumption is correct that there is more attention devoted to Mary (some might feel that Abigail's presence is more looming throughout the play, as her actions caused in motion the events of the play), it might be due in part to what Mary represents. Miller's characterization of Mary Warren creates the understanding of what the townspeople of Salem are like. In a sense, Mary represents the democratic essence of the town. Her timidity, freight, and eventual hysterics in court prove that the democratic experience is not one for the faint or meek. Rather, the government of the people must be comprised of individuals who possess the moral fortitude to voice their own sense of activism and stand in defense of the public good. Miller's creation of Salem represents the failure of the democratic experiment when malevolent forces combine with a public fear of voicing opposition. Abigail represents these malevolent forces with her accusations that she knows to be false and her desiring of self- interested end with her accusations. Yet, Miller is making a critical point about the failures in democracy in that it is not merely the presence of evil which dooms such a system, but rather its convergence with the absence of good. The failure of democracy in Salem, and in general, happens when both malice and silence work in tandem with one another. Abigail represents one obvious element, yet the exploration of Mary's character might be the delving into the other.
I have a question! What was the real reason that Mary Warren was absent from court during the previous week?