Why people are differentiated as poor and rich?

One reason is because of capitalism itself. All forms of economics serve, in a sense, to ration food and supplies. Capitalism does so by ensuring that you contribute to society by working in a job. That job pays you with a certain amount of money you can use to buy food and supplies. Different jobs get different pays. That way, you earn your share.

However, it is the case that many people can't even get jobs. As a result, they don't make money and can't buy food and supplies (including shelter even). There are also some who do have jobs (such as McDonald's minimum wage jobs) that still don't make enough money to sustain themselves.

Because of this, there is much more money still in the workplace, so to speak. That money is taken by the rich: people who decide the salaries, or people who fill in spots with exceedingly high salaries. They get a lot of money, and therefore, a lot of food and supplies.

Capitalism, like all others, seeks to ration food and supplies, and it does, but not in the best way.