Each of these two questions relates to the other, according to Francis Nataf. In one talmudic passage, God threatens to drop Mount Sinai on the heads of the Israelites if they do not accept the Torah; in a second passage, God chooses the Jews to receive the Torah because of their stubbornness. Over the centuries, rabbis have tried to synthesize the two:
[The 18th-century rabbi] Yaakov Yehoshua Falk . . . explains that the stubborn (or, better, “determined” or “brazen”) nature of the Jews is actually the reason that it made sense to force them [to accept the Torah]. . . . [I]t was only due to the characteristic determination of the Jews that they would subsequently . . . delve into the Torah they had received by force, and eventually accept it voluntarily (as the talmudic sages say happened subsequently). . . . According to Falk, if coercion can eventually lead to voluntary observance, it would be not only legitimate but absolutely necessary. . . .
But why, then, does God not force the Torah on everyone? Nataf continues:
Perhaps . . . the Jews are made the agent of bringing about an engagement of mankind with God’s will, an engagement that can eventually lead to voluntary acceptance. . . . [T]he coercion of the Jews can get the ball rolling for non-Jews as well; once the Jews are involved with Torah and accept it, the Torah—with its highly unusual people—piques the curiosity of all those who come into contact with it. . . . And the march of Western history has shown that, like it or not, the Gentiles have responded to the call of the “God of the Jews.”
Read more on Torah Musings: http://www.torahmusings.com/2015/05/why-werent-the-non-jews-under-the-mountain/