Parshanut к Шмот 34:39

And no man will covet. When the three pilgrim visits are mentioned in Parashas Mishpatim (23:17) this promise does not appear. This is because before the sin of the calf the B’nei Yisrael were completely unassailable. Now, however, they were vulnerable to attack and needed special assurance that they would come to no harm through making the pilgrimage (Meshech Chochmah).