First, let's ask what relativism means. The usual understanding is that it says what's right and wrong is not universal, but relative to some non-universal reference point—the predominant opinions in one's culture, typically.
Your question appears to assume that relativism is the only good explanation for differences in behavior, but it's not clear why we should believe that. After all, many differences in behavior are matters of preference. I prefer to eat chocolate ice cream; you like rum and raisin. Neither of us is wrong, and relativism is neither relevant nor useful in explaining the difference between us. I like swing dancing; you don't. I don't like playing basketball; you do. We'll behave differently on that account. But neither of us is "right" or "wrong," and once again, relativism doesn't provide any additional insight. Wh do our taste in ice cream differ? Why do we prefer different leisure activities? Who knows? The answer is probably a complicated mixture of a lot of things, some having to do with our individual histories, some with our neurological wiring, and others to do with who knows what. But we've gotten nowhere near what's usually called relativism.
The reply might be that I'm being obtuse. When you ask why people behave differently, you're asking about differences in behavior around matters we take to have a moral dimension. So let's focus there.
What people think is right and wrong is sometimes not well-thought-out. For centuries, most people thought that homosexuality was wrong. But what they thought didn't make it so, and when we look at the question carefully, we see that the age-old opinion was based on nothing that stands up to scrutiny. The fact that some cultures don't see it this way is, indeed, a fact, but the predominant view of a culture on some moral matter can be just plain wrong. Enslaving people was wrong even when it was common, and is wrong even where it's practiced now. This isn't an arbitrary dictum. We can say why slavery is wrong. It's cruel, it's degrading, and it treats people in ways we wouldn't find it acceptable to be treated ourselves. Morality isn't arbitrary. Moral claims aren't (or shouldn't be) just made up. We can reason about moral matters, and those reasons, rather than often unexamined norms, are what matter.
That said, some things that are morally significant can be relative in a certain sense. Even if treating people with respect is always important, how respect is expressed and what counts as respectful behavior depends partly on custom. The customs shape the meaning of the behavior in the cultural context, and the meaning of one's behavior matters morally. But this kind of relativism is not what people who defend relativism usually have in mind.
To take a related example: in the USA, it's wrong and not just illegal to drive on the left side of the highway. In South Africa, it's wrong and not just illegal to drive on the right. Which side we drive on matters because lives are literally at stake. But there's no intrinsically right way to pick the convention (for it is a convention) that a country uses. When I've driven in South Africa, I've driven on the left for the same reason I drive on the right here at home: I don't want to put people (myself included) in danger. Even though "right side of the road to drive on") is relative to an arbitrary choice, once the choice has been made then there's nothing "relative" about whether one should follow it.
Of course, this sort of "relativism" does help explain some (by no means all) differences in behavior. But it's a "relativism" that's ultimately in service of something that's not relative at all.