In the Pesukim introducing the Aseret Hadibrot the Pasuk says, ועתה אם שמוע תשמעו בקול. Why specifically now? Why only from now on are they are they asked to listen to Hashem? The שם משמואל answers that a person can only have complete קבלת עול מלכות שמים when he is free, when he is not controlled by someone else. The Yerushalmi explains that an עבד כנעני is exempt from reciting קריאת שמע because since שמע is קבלת עול מלכות שמים and he is under someone else’s control other than Hashem’s, he cannot recite שמע properly. It is impossible to accept עול מלכות שמים in its entirety when Hashem is not the sole ruler over you. Therefore, the reason the Pasuk has to say ועתה is because after they left Egypt they are now totally משועבד to Hashem and they are capable of acquiring the עול מלכות שמים.
Another possible explanation is based on Bnai Yisrael’s actions at Masa Umerivah. At the end of Parshat Beshalach Bnai Yisrael call the place where they wereמסה ומריבה על ריב בני ישראל ועל נסתם את ה' לאמר היש ה' בקרבנו אם אין. They called it Masa Umerivah because of the striving of Bnai Yisrael and because they tried Hashem saying is Hashem among us or not. Soon after that episode, Bnai Yisrael encounter Amalek and Hashem answers Moshe’s Tefilot and Bnai Yisrael defeat Amalek. Hashem’s answer to Bnai Yisrael’s questionהיש ה' בקרבנו אם אין is the defeat of Amalek. For this reason Hashem says, ועתה to say that now that I have answered your question now you should listen to me and do my Mitzvot. It is only after this that Bnai Yisrael will become a ממלכת כהנים וגוי קדוש.