The Great Migration is the annual migration of wildebeest, along with other grazers such as zebra’s & gazelles. Each year they migrate from the Ndutu region of the Ngorongoro Crater in Tanzania, trough the great plains of the Serengeti, to the Maasai Mara in southern Kenya .
Over two million (yes million) individuals make the earth tremble as they migrate trough Tanzania’s lands in search of fresh food. Opposed to what some people think, you’re able to see the great migration in Tanzania year round. There are however a few months where many of the animals of the migration are scattered troughout Serengeti national park, making it less spectacular than seeing them all together.
To keep the answer very simple: they are in search for fresh food. The wildebeest migrate the way they do, because they follow the rainfall. The rains greatly influence the availability of fresh grasses and other food sources, so the wildebeest follow.
If the rain season is unpredictable, the great migration might be just as unpredictable. They go in between Ndutu and the Northern parts of Serengeti wherever the grass is greener.
More rain can cause the migration to migrate slower, while a drought might cause the migration to reach he sountern Ndutu or the Mara river faster.
Because of the plentiness of rainfall these last years, not all wildebeest decide to cross the mara river, risking their lives. So some of them stay behind as there’s enough fresh food on the other side. Why risk your life if you don’t have to ? The great migration rarely fully reaches the Masai Mara in Kenya, because there’s enough food available on the other side.