Why did mecca want to silence Muhammad?

Because Muhammad taught in the name of Islam that there was only one God, Allah, and the Meccans were polytheistic meaning that they worshipped many gods.

The Arabs in Mecca, known as the tribe of Quraish, used to make a living on the business they would make when people from foreign lands came to make pilgrimage in their city to the idol gods. Muhammad was seen as a clear threat as he said that the idols weren't worthy of worship, and as such, the people of Quraysh who hadn't accepted Islam wished to kill him.