The Arugas Habosem offers a profound answer to this question. He says that due to the close-knit unity amongst them, even the Jews who were free felt the suffering of their fellow brethren who were working during their shifts. So much so, that they felt the pain as if it were their own, for they were as one body. Thus, although not all the Jews were working at the same time in the literal sense, they were still unable to find rest as long as their fellow Jews were being enslaved. Their unity caused them to intensely feel their fellow Jews' anguish and thereby accentuated their own suffering.
This immense unity, which came about through their deep love for each other, is why they merited their redemption. Abstaining from slander was a result of the deep sense of unity and solidarity that the Yidden experienced during their time in Egypt. This unity created a strong bond of loyalty among them, with each person sharing in the pain of their fellow Jews and going out of their way to provide assistance whenever possible.
By refraining from negative speech, they demonstrated their commitment to this unity. Ultimately, it was this unity, which had great significance in the Eyes of Hashem, that played a key role in their redemption from slavery in Egypt.
(ערוגת הבשם פר' וארא ד"ה ועד"ז)