Why did the cattle stock of the Maasai's decrease under colonial rule?

Why did the cattle stock of the Maasai’s decrease under colonial rule?

In the 19th century, Maasai tribes could move over vast areas in search of pastures. From the late 19th century, the Colonial Government began imposing various restrictions on their mobility. The Maasais were bound down to a fixed area. They were cut off from the best grazing lands and forced to live within a semi-arid tract prone to frequent droughts.
Since, they could not shift their cattle to places where pastures were available, large numbers of Maasai cattle died of starvation and disease. As the area of grazing lands decreased, the adverse effect of the drought increased.
In just 2 years of severe drought 1933 and 1934, over half of the cattle of Maasai tribes died. In this way, the cattle stock of the Maasai’s decreased under colonial rule.