Juliet lived in a deeply patriarchal society, one where women had very little agency in their own personal lives. You can see this power imbalance in act 3, scene 5, when she attempts to resist her father's insistence that she marry Paris. Enraged, Capulet threatens to disown her and throw her into the street.
Capulet's reaction is in response to Juliet's waffling about a marriage to Paris; with that in mind, how should she expect her father to respond to the news that she has already been married (having done so in secrecy), and to a Montague at that? If Romeo and Juliet were going to reveal their marriage publicly, it would have probably been preferable to do so as soon as possible (especially given the instability of the feud), but after the events of act 3, scene 1, with Romeo's killing of Tybalt in a duel, that solution has largely become untenable. Romeo has killed Juliet's cousin and, even more than before, established himself (on his own merits rather than as an extension of his family) as an enemy to the Capulet family. With that in mind, seen from Juliet's perspective, by this point it would have been too late to reveal the truth of her marriage. Overwhelmed, she is trapped in a situation that has spiraled out of control.
Juliet is fearful of her parents, especially her father, who reacts very harshly when she says she does not want to marry Paris. He has already contracted for the match, feels he is doing her a favor, and expects her to obey him. Moreover, he does not want to lose face in front of Paris from backing out of the match, which would be humiliating to him at this point.
Juliet is fearful, but when she turns to her nurse, she feels isolated and unsupported, for her nurse advises her to be practical and commit bigamy by marrying Paris. The nurse also belittles Romeo. Juliet, at this point, is desperate and feels misunderstood.
When Juliet goes to the friar for advice, he offers guidance that is more designed to save him from exposure and wrath than advice that will genuinely help Juliet. He should have told her to come clean and go her parents and confess the truth, but instead, he contrives another coverup, in which she will take a drug that feigns death. He persuades Juliet, against her best interest, to go along with this complicated plan. He hopes this way that he can keep his secret marriage of the two a secret for sometime longer.
Juliet is betrayed on all sides by adults: her father with his harsh and punitive manner, her overly pragmatic nurse, and her self-interested friar. Romeo is not around to help, and Juliet makes a poor decision out of fear and confusion, not lack of love for Romeo.
Juliet would never tell her parents of her marriage because she has toally alienated her parents from her decision to marry Romeo. This was unheard of in Elizabethean Times.Marriages were arranged by the parents ,usually from birth or at a very early age, and there was a process to follow. The gentleman was to ask the father's permission to marry, court the lady, and then get the final approval of her father and mother for the marriage to occur. Count Paris has done all those steps. Romeo is an enemy of the Capulet family because of a feud that has nothing to do with him. He is an outcast before he could ever ask for a chance. I also feel that Juliet is afraid of her father and this is supported by his outrage, verbally and physically, in Act III Scene 5 when Juliet refuses to marry Paris. Juliet feels that the only way to be with the man she loves is to risk it all and do whatever is necessary to live forever with her Romeo. Death doesn't matter at this point as long as her love continues.
At the beginning of the play, Juliet illustrates the expected level of obedience to her parents in who she should marry when she responds her mother's question of whether she could love Paris in Act I, Scene 3: "I'll look to like, if looking liking move: But no more deep will I endart mine eye/ Than your consent gives strength to make it fly". Juliet would only allow her feeling to go as far as her parents would allow and consent to.
Her parent's reaction to the Juliet's declaration that she will not marry Paris on Thursday in Act III, Scene 5 shows the reactions and consequences of Juliet's disobedience to her parent's wishes.
Lord Capulet: "Hang thee, young baggage! disobedient wretch!/ I tell thee what,--get thee to church o' Thursday, /Or never after look me in the face:/ Speak not, reply not, do not answer me;/ My fingers itch.--Wife, we scarce thought us bles'd/ That God had lent us but this only child;/ But now I see this one is one too much,/ And that we have a curse in having her: Out on her, hilding!"
Lady Capulet: "Talk not to me, for I'll not speak a word;/ Do as thou wilt, for I have done with thee".
Both Lord and Lady Capulet are willing to disown Juliet helps to illustrate why Juliet would be unwilling to tell her parents that she had not only married against their wishes, but to the son of her family's mortal enemy.