There's a well known midrash that HaShem went around offering the Torah to other nations prior to offering it to Israel, with the nations rejecting it for various reasons (restrictions on theft, murder and adultery).
Weren't they already obligated in those commandments as bnei noach (if anything it would get better for them as it is easier to convict under bnei noach law)?
I remember hearing from Rav Yaakov Weinberg z"l, something like:
The Sheva Mitzvos for Bnei Noach are not a ברית, a covenant. They don't define a relationship. They are just a set of rules posted on the wall of the hotel: If you stay here you must do the following...
The Torah is very different. They were being offered the opportunity to be part of the ברית with Hashem, with Israel. They would have had a relationship.
I don't think they were offered the same relationship as Israel - my clear impression from the Sifri on V'zos Habracha is that they were bound to be in a less central position (see the part about the donkey and the dog). The Midrash sounds like they would not have had 613 mitzvos; from the Midrash, more than just 7 mitzvos, but less than what Israel was given. I don't know what it would have looked like exactly. Maybe each nation would have emphasized different things?
But they were offered a real covenant, and the world would be a better place if they had joined. We would not have to wait until Moshiach for all the nations to serve Hashem.
So I think the answer to your question is that they never really had to accept it before. Now it was real, and they weren't willing to do that.
There are many answers given to this question. Among them are:
The scope of mitzvos is different: For an aku"m, murder means unlawfully killing another person. Yet, for a Jew, murder goes far beyond that. Someone who embarrasses his friend publicly, malbin pnei chaveiro b'rabim, is guilty according to Chazal of the dust of shefichus damim, murder. The same is true of arayos, theft, and many other mitzvos. You don't need to mug someone to be guilty of gezel. If you make noise that disturbs your neighbor's sleep, it's gezel sheinah. This is what the nations of the world were rejecting.
Torah is a lifestlye, not just a set of laws. What Eisav and Yishmael and the other nations were telling G-d is that their culture is one of bloodshed, theft, arayos, etc. The ben Noach laws require them to curb their natural instinct, but Torah isn't only about that. The Rambam writes in Shmoneh Perakim that the goal of Torah is to transform a person into someone who does not desire bloodshed, or theft, or other crimes -- not simply to avoid committing them. The nations could not envision changing in that way
It's true that the Bnei Noach were already commanded in those laws. But here they were being asked: "Do you WANT to accept these laws?" Their response was in the negative.
