Or HaChaim on Numbers 12:5/אור החיים על במדבר י״ב:ה׳

1 וירד ה׳ בעמוד הענן, G'd descended within a pillar of cloud. We need to know why G'd did not first descend prior to His calling upon Moses, Aaron and Miriam. At least it appears from the order of our text that He waited with descending until after He had called to them. The fact that G'd told the three to leave their tents and come to the Tabernacle also suggests that He had not yet descended, otherwise he would have had to assign a certain place for them as they would have observed the place where the cloud had taken up position. According to the scholars who hold that both Aaron and Miriam were ritually impure at the time, we can well understand why G'd waited before He descended in His cloud. He did not want to descend and then have to wait until Aaron and Miriam had undergone their purification rites. It might also make those who had been called and had not responded promptly subject to posssible punishment. As it is not. G'd's wish to do something which results in people being punished, He decided to wait until both Aaron and Miriam were able to respond to His call. The reason G'd descended at all and did not address them by His voice emanating from higher regions as He had done when He first called out to them was in order to lend publicity to the event. When they would see the cloud take up position at the entrance of the Tabernacle all of Israel would become aware that G'd had been greatly concerned that someone had imputed base motives to His trusted servant Moses and that He had punished that someone.