Cruelty or selfishness would be the most obvious reasons. But I don't think it's either of those. My guess is that he's just blinded by his family's opinions, preferences and plans and tends to forget that you exist.
So even though he's treating you like crap when you're with his family, at least he's not purposefully trying to treat you like crap. Does that make the situation better? Not really, but it does mean there's something you can do to about it.
If your boyfriend was just malicious or self-centered, then there would be very little you could do to change it. But I think he just has no idea how you feel when you're with his family. You feel steamrolled and powerless, so you need to tell him that. Sit him down and have a conversation.
Don't say that you hate his family, because that will make him defensive. Instead, just tell him that you feel ignored when you're with them. Say that there are times that you don't want to do exactly what they want to do, and that you'd like him to understand that. If he is a decent and empathetic guy, he's going to change his behavior going forward, and stick up for you when you want time away from his family.
Best of luck, and let us know how it goes. Ladies, have you been in a similar situation?