five pillars

Islam is based on five pillars, which the Messenger, may God bless him and grant him peace, said:
Islam is based on five: testifying that there is no god but Allah and that Muhammad is the Messenger of God, establishing prayer, paying zakat, fasting Ramadan, and making the pilgrimage to the House-for those who can do so-

why?

Islam is a belief in which God and His Messenger have defined what is permissible and forbidden, morals, worship, transactions, rights, and duties.
And God was pleased that Sharia became the way of life for humanity until the Day of Resurrection.