1.One of the most prominent reason for this is...In 1885 Germany and England clashed for land possesion in Africa and ended up dividing the land into Colonial Kenya and German Tanzania, English took most of the land and Massai people were confined in the semi arid tract of south kenya and Tanzania.
2. Droughts made the condition worse.
3.Reservation of forest
4.Imposing tax on cattle
5.Large part of grazing ground were converted into Park and sancturies.