MJ requested the injection because he couldn't sleep and wanted to use it to supplement his sleep. But as a doctor, Murray shouldn't have provided it, especially without any monitoring equipment, and isoproterenol, a powerful anesthetic, should only be used in the operating room. The evidence suggests that Murray was in a financial crisis at the time, and it is supposedly the high compensation that made him compromise on MJ's unreasonable demands and throw medical codes out the window. Patients don't know any better, doctors can't afford not to. During the dangerous injection, Murray actually left MJ's side to text and call his girlfriend, and failed to take the right steps to save MJ's life when something happened to him.
So the reason for this is that Murray is a medically deficient and irresponsible person.
Some other conspiracy theories say that Murray was hired by certain powers in the entertainment industry to murder MJ, these stories are just speculation, it doesn't hurt to look at them, believe it or not.