First, Britain had a lot of advantages for fighting in war such as, a lot of more advanced weapons and a well-trained military. The majority of the Americans that were fighting were merely farmers who had never held a gun in their life. So, some American colonists would be a loyalist because they didn't believe America stood a chance against Great Britain.
Another reason was because they feared the punishments of King George. When speaking to the British Parliament, King George III stated that the all the American colonists were traitors who were committing treason, and the consequences of continuing such acts would be death, etc.. Some Americans chose to be loyalists because the feared the punishments that traitors were subject to, death in particular.