Answer:
Becuse they didn't believe in his mericlies and thought he was asscoating with the devil, Even though he gave all the evidence. They also envied him becuse of his followings and his honesty. They were frustrated because he wouldn't give up and always had a positive outlook on things as like as he is being led by a bigger and much greater entity so they were against him and wouldn't sell him a thing.
The Quran asserts that Muhammad was a man who possessed the highest moral excellence, and that God made him a good example or a "goodly model" for Muslims to follow (Quran 68:4, and 33:21). The Quran disclaims any superhuman characteristics for Muhammad, but describes him in terms of positive human qualities.
In the Quran there are words from God as all the Muslims believe and obey by out of love and that's what Mohammed pbu used to preach.