At Mount Sinai, God did not descend immediately because Israel needed time to prepare.
Now let's view the situation, as best we can, from GOD's viewpoint.
Why did GOD wait three days? Israel accepted the terms of the covenant agreement, that they would obey Him and He alone would be their God.
GOD waited three days before descending on Mount Sinai because the children of Israel were not ready. They needed time to prepare themselves.
3 Then Moses went up to the mountain of God, and God called to him from the mountain, saying, “Thus you shall say to the house of Jacob and tell the children of Israel: 4 ‘You have seen what I did to the Egyptians, and how I bore you on eagles’ wings and brought you to Myself. 5 Now therefore, if you will indeed obey My voice and keep My covenant, you shall be a special people to Me above all nations; for all the earth is Mine. 6 You shall be to Me a royal priesthood and a holy nation.’ These are the words you shall speak to the children of Israel.”
7 So Moses came and called for the elders of the people, and laid before them all these words the Lord commanded him. 8 Then all the people answered with one accord and said, “Everything the Lord said we will do and hearken to.” So Moses reported these words to God. 9 The Lord then said to Moses, “Behold, I come to you in the pillar of cloud, that the people may hear when I speak with you and believe you forever.” So Moses reported the words of the people to the Lord.
10 Again the Lord said to Moses, “Go down and solemnly charge the people and sanctify them today and tomorrow, and let them wash their clothes. 11 Let them be ready for the third day, for on the third day the Lord will descend upon Mount Sinai in the sight of all the people. 12 You shall set bounds for the people all around, saying, ‘Take heed to yourselves that you do not go up to the mountain or touch its base. Whoever touches the mountain shall surely die. 13 Not a hand shall touch him, but he shall surely be stoned or shot with an arrow; whether man or beast, he shall not live.’ When the thunders, the trumpets, and the cloud depart from the mountain, they shall ascend the mountain.”
(Exodus 19:3-13 OSB)
GOD did not come for His children until the time was right, until they were ready, until they had sanctified themselves.