Man has free will. As a result, Hashem has to allow man to decide for himself what course he will take. However, the conditions before the flood were such as to not allow the rebellion of the youth to overcome the evil customs and behavior that were the status quo. Rav Hirsch in his commentary on Noach 8:21 points out that now, after the flood, the spirit of rebellion of the youth would now allow them to bring the world away from evil as well as towards evil. Rav Hirsch uses the analogy of Hashem specifically having picked the most stiff-necked and stubborn people to receive the Torah.
Similarly, Hashem punished the generation of the dispersion (as explained by Rav Hirsch) by making the traits of rebellion and the development of local dialects so extreme as to change the languages that people spoke at once instead of over the course of generations. Thus, the society would have broken up anyway, but Hashem used that trait to prevent the complete destruction that would have resulted from the ant hill that the civilization was becoming.
The people are not the same after the flood as before the flood. The lifespan of centuries is reduced to decades so that the new generation can take over before the evil becomes totally uniform. The evil as it was before the flood can no longer cover the entire world as a uniform set of evil tenets.
Rav Hirsch points out in 8:22 that the new climate and geographical conditions forced by the flood also leads to the dispersion and independence of societies so that no single society can conquer the world and spread evil over the entire habitat of mankind.
