Before offering the Torah to Israel, God first approached every other nation on earth. The Mekhilta DeRabbi Yishmael explains that this was not because God expected them to accept. It was a preemptive legal maneuver.
If God had given the Torah directly to Israel without asking anyone else, the nations of the world would have had a legitimate complaint. They could have stood before the Shechinah, the divine presence, and argued: "Had we been asked, we would have accepted it. You never gave us a chance." This would have been a valid grievance, and God chose to eliminate it entirely.
So He solicited every nation. He went from people to people, offering them the Torah and its commandments. And every single one of them refused. They were asked, they heard the terms, and they said no.
The proof text comes from (Deuteronomy 33:2), which describes the Lord coming "from Sinai" and shining forth from various directions, from Seir, from Mount Paran, approaching from different lands before arriving at Sinai. The rabbis read this geographically: God traveled to the territories of different nations, offering His Torah to each in turn.
The result is that no nation can ever claim they were overlooked. They were all solicited. They all declined. Israel alone said yes. The covenant at Sinai was not an arbitrary selection. It was the outcome of a universal offer that only one people accepted.