it is basically because of unequal distribution of resources. Some people have more they need and others lack the basic necessities of life. The rich don't worry about the poor, they mainly focus upon increasing their own wealth. The poor people struggle to even get a single meal a day. A state has enough to provide to every citizen but they normally are not willing to do this. Rich people need more.
Poverty is one of the greatest problem in the world which almost every country is facing. The common cause is corruption. Politicians and rich people are stealing money without thinking that their public is dying of hunger. This corruption leads to unequal distribution of money because money lies only in few hands, the result is then unequal distribution of resources.