The short answer is that they were defeated by the British in a bloody 5 month war.

Historically, the Zulus were much weakened from the time of Shaka. The Mfecane had reduced the population of the entire region by as much as one or two million people. Further, there had been a number of purges from the 1840s before Cetshwayo came to power (C pronounced with a frontal palate click). This and the associated dissatisfaction caused some to leave. So Cetshwayo took over a significantly weakened state. Finally, the Boers had been pushing into Natal and the Southern sub-continent, causing pressure and further weakening the Zulus.

How could they have survived? Hard to say, but I think a clear, uncontested line of succession from Shaka onwards would have made a huge difference.