Seforno: It was due to the fact that He was about to speak to them firsthand. Consequently, He was afraid that they would consider themselves on a par with Moshe Rabeinu, and ascend the mountain in order to join him.

Oznayim la'Torah #1: To teach us that one needs to warn immediately prior to the act. ¹

Oznayim la'Torah #2: Because so powerful was the desire to see (even what was forbidden) that some people would break their ranks to see - in spite of the severe repercussions.

Da'as Chochmah u'Musar Vol. 1, p. 53: Because even Kohanim at the highest level of closeness to Hashem are prone to want to show that they are more important, and one may not take Chochmah for (to aggrandize) oneself!

Because 'man was created to enjoy the radiance of the Shechinah,' and Ma'amad Har Sinai was the greatest revelation of the Shechinah in this world ever. Consequently, for such a spiritual Hana'ah, people were prone to do an Aveirah Lishmah. ² (PF).

Rashi: Because if even one Jew falls, it is as if many fell.

Targum Yonasan: Even a great man should not think that Hashem will turn a blind eye if he ascends the mountain ¹ - because even he will fall.