To answer this question suitably, one must first understand the profoundly deep Christian ethic which dominated Salem society. Salem was ruled by a theocracy, a society that believed in the rigid adherence to all things Biblical. Every form of pleasure was discouraged and even punished and there were strict rules about attending church, knowing scripture and so on.
This rigidity was borne out of a past in which the inhabitants of the New World's forefathers themselves had been persecuted in their home country. It was therefore essential for them to fight anything which they deemed evil and that could be done only through rigid control and a strict adherence to biblical laws. This would ensure their safety and free them from bondage.
The environment in which the people of Salem lived in, was therefore, a society ruled by fear and suspicion. The mood was, as a result, generally sombre. Inhabitants found comfort in their belief that their lives and their destinies were ruled by God and it was their duty to live out their time on earth according to His will. Overstepping the boundaries in this sense then, could result in severe sanction. They dedicated their lives to hard work, which for them was a means of staying out of trouble. Working hard to them meant that there was less time to be enticed into doing wrong since, 'idle hands are the devil's playthings.'
It is within this background that the Salem folk believed that Satan was a constant threat and that he had to be fought or warded off at any cost. Anyone outside their rigidly structured society was fundamentally seen as a threat. The surrounding forests were seen as place in which denizens of evil dwelt, as the author himself explains in his introduction:
... very few Indians were converted, and the Salem folk believed that the virgin forest was the Devil’s last preserve, his home base and the citadel of his final stand. To the best of their knowledge the American forest was the last place on earth that was not paying homage to God.
In this regard then, it became essential for the inhabitants of this Puritanical society to, as it were, 'gang together' to defend themselves against the threat of evil. They believed that the Indians were savage heathens and the devil's disciples. Proof for this belief, they thought, was confirmed by the spurious attacks on homesteads and farmhouses by the Indians.
Another reason why the Indians could not be converted, was the issue of 'parochial snobbery.' The inhabitants of Salem were largely an exclusive society and it was not their purpose to go out and convert. It was safer, they believed, to be with their own than to risk contact with outsiders, especially those whom they mistrusted and feared so emphatically. As Arthur Miller states:
The parochial snobbery of these people was partly responsible for their failure to convert the Indians. Probably they also preferred to take land from heathens rather than from fellow Christians.
Furthermore, taking land from the heathen savages, as Miller suggests, was a better option than taking land from one's own. This was probably seen as a victory against Satan and his disciples.