Why can't he just stop liking girls pictures?

My boyfriend won't stop liking pictures of half naked girls. Even though he knows that it is causing problems between us and how much it bothers me. Its the only issue in our relationship. Whenever I confront him and tell him how it makes me feel, he always promises me that he will stop and says that I don't deserve to feel like this and that he's sorry. It always seems sincre but he doesn't stop. We've been together a year and I've been talking to him about this since we started dating. I feel like it's more important to him to like girls pictures then how it makes me feel. And It sucks when someone promises you something over and over but never keeps their promises. I've tried to not let it bother me but it really does. I don't understand why he won't just stop.
Why can't he just stop liking girls pictures?
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