Why is the green card process so long and complex for people from India and China?

The US Immigration system issues Permanent Residence cards, also known as green cards, which lets people reside and work on a permanent basis in the United States. The number of green cards issued each year is fixed at 140000 and each country is capped with a 7% quota i.e. no more than 9800 green cards can be issued to residents of a single country. Since there are hundreds of thousands of people who come to work in the US from India and China every year, they have to wait several years before they become eligible for a green card. This long delay can disrupt the lives of these people since the work visa they have does not grant them permission to reside and work in the US on a permanent basis, and they may have to leave the US, along with their families, if their work visa expires.