Because they stopped in their migration.......

"Allowing that the human race multiplied on the earth after the flood in the ratio that the Israelites did in Egypt, their numbers would double once in fourteen years, or seventeen times in two hundred and forty years. 

---If so, they would number at this time more than a million of souls,-a number sufficiently large to begin to scatter abroad over the face of the earth. 

The division of the nations "in the earth after the flood" was divinely appointed; for God "made of one blood all nations of men for to dwell on all the face of the earth, and hath determined the times before appointed, and the bounds of their habitation." 

Genesis 10:32/Acts 17:26. 

To this division some rebelled; for we read: "And it came to pass, as

they journeyed from the east, that they found a plain in the land of Shinar; and they dwelt there. . . . And they said, Go to, let us build us a city, and a tower, whose top may reach unto heaven; and let us make us a name, lest we be scattered abroad upon the face of the whole earth." Genesis 11:2, 4. 

The leader in this rebellion is supposed to have been Nimrod, whose name signifies the rebellious. 

He was the Belus of the ancients, a great-grandson of Noah, being the youngest son of Cush, a son of Ham. "He began to be a mighty one in the earth. . . . And the beginning of his kingdom was Babel . . . in the land of Shinar." Genesis 10:8, 10. 

Because they stopped in their migration, God confounded their language, that they might not "understand one another's speech. . . . So the Lord scattered them abroad from thence upon the face of all the earth; and they left off to build the city. Therefore is the name of it called Babel; . . . and from thence did the Lord scatter them abroad upon the face of all the earth." Genesis 11:7-9."
Sylvester Bliss