Or HaChaim on Numbers 12:4/אור החיים על במדבר י״ב:ד׳

1אל משה, to Moses, etc. The reason that G'd called to Moses when it was Aaron and Miriam He wanted to speak to is that the latter were ritually impure at the time and first had to purify themselves. He called to Moses in order to give recognition to the fact that Moses kept himself pure at all times by the very fact that he did not indulge in marital relations with his wife. This is why he was able to be called upon by G'd without having to undergo any preparation. This fact contrasted with the state of Miriam and Aaron (compare Yalkut). According to the plain meaning of the verse G'd called Moses so that he would be able to offer a prayer on behalf of Miriam when the latter would be afflicted with Tzoraat. Another reason G'd called Moses also was that G'd wanted to use the opportunity to demonstrate Moses' superiority as a prophet immediately. He did this by calling to Moses before He called to Aaron and Miriam. When they would hear that G'd called Moses first, they would realise immediately that this was in contrast with their perception that they equalled him in prophetic stature.