The 1st Chabad Rebbe, Rabbi Shneur Zalman of Liadi, elucidates why teshuva is possible in this world:[1]
Because this world is an admixture of good and bad it is therefore possible—through teshuvah—to transform bad to good.
This can be compared to the child in the first 40 days of the gestation period. At that period, the head and the leg are combined and encompassed in the seed, and the undeveloped fetus can be altered in various ways. It is because of this, that in that time, prayer for a male is affective, (while afterwards it is futile).
In the world to come, good and evil are separated from one another—it is therefore impossible to transform bad into good.
That stage is akin to the fetus after the initial 40 days of gestation—where at that point the head cannot be transformed into a foot.
[1] Likutei Torah, Pinchas, 75c
