The question has two parts:
How and why
Regarding how prophet Muhammad (PBUH) began a new religion:
It was Allah (God in English) will to conclude all previous messages of God prophets with the last message of prophet Muhammad. So, it is not prophet Muhammad who started the new religion but it was God will and command to him to convey the message of Islam to humankind through the God revelation of Quran; that reflects real God words in its real revelation language (Arabic) to prophet Muhammad through the angel Gabriel (Jibril).
Regarding why prophet Muhammad (PBUH) began a new religion:
Islam is not a new religion. It is the religion since the start of universe creation and was the call of all God prophets although named differently by different prophets. Refer to related question below. However, Islam per Quran revelation to prophet Muhammad (PBUH) was to correct for the deviations and alterations that were introduced in previous holy books by some people to fix certain beliefs and ideas and for other reasons that could be referred to in the related question below on why was Quran revealed.
Hazrat Muhammad didn't start preaching Islam of his own accord. He was commanded to do so by Almighty God through Arch Angel Gabriel. Islam was neither FOUNDED nor DEVELOPED by some person. Islam is the religion given to mankind by the Creator, Almighty God. All the holy Prophets (May peace be upon them all) preached Islam. The last Prophet Hazrat Muhammad Mustafa (SAW) started preaching Islam at the age of 40, in 570 AD in Makka, Arabia. Islam is based on human nature and preaches universal human brotherhood.The basic belief of Islam is ONENESS of Almighty God Who is the Sole Creator, Sustainer, Provider and Ruler of all the worlds. He has no Son, no Parents, No Partner. He is Unique, Nothing is like Him. He is the CREATOR, all others are His CREATURES.
Islam
Muhammad started teaching Islam religion in the year of 610 A.D
739 ad.
Islam
At first most of them did not. But with time, and after witnessing the ethics of people who followed Muhammad, almost all of them changed their mind and join the new religion.
When he received the message from God, when the people had been very from God.
A:Muslims believe that the angel Gabriel (Jibrel) came to Muhammad. Islam teaches that Muhammad did not so much start a new religion as reveal ancient truths on which Judaism and Christianity were already based.
Islam is the religion based on the teaching of Muhammad.
The religion that Muhammad is favored is the Islamic religion. The Islamic religion regards Muhammad as its most important prophet, and pilgrims to sites he visited are important parts of the religion.
He was sad and made a new religion, Islam
Muhammad did not develop any religion. He was a prophet of God, and he conveyed the religion of Islam.
The religion of Islam was founded by the Arabian Prophet Muhammad in 621 CE.