The best way to answer this question is to put yourself in the shoes of the poor unfortunates who happen to encounter Frankenstein's monster. How would you feel if you saw this big ugly creature coming towards you? Most people would be scared out of their wits, and rightly so. It's fair to say that no one's ever seen such a strange creature in their whole lives, so it's only natural that they respond to the monster by treating him like a monster.
Also, one should bear in mind the fact that the monster behaves in an appropriately monstrous manner. This is a creature that actually kills people and doesn't appear to have a conscience. For most people, that would seem to be the very definition of a monster.
Later on, the monster starts becoming more of a sympathetic character. He wants to be a human; he no longer wants to be a crazed, savage monster; he reads Paradise Lost.
But it's too little, too late. The monster cannot change his past. Nor, for that matter, can he change his appearance. He still looks utterly hideous. As soon as anyone sees him coming, they're likely to run to the hills, screaming. While it might be a nice idea to treat the monster like a human being, it's not very realistic.