why does the creature calls the cottagers his protectors

As he tells his story to Victor, the monster admits that he liked to call the De Laceys his "protectors" even though they were unaware of his presence. He says that it was "an innocent, half painful self-deceit" to call them by this name. The irony, of course, is that once the creature is discovered by the De Laceys, Felix attacks him rather than protecting him.

He pretends that he is a part of their family and that they would take care of him and be his protectors. He wants to be accepted.