R. Shlomo Kluger answered this question at the beginning of parshas Tzav in his explanation as to why the parsha started with the Olah offering (the burnt offering):
The Midrash in parshas Beha’alosecha on the posuk “take the Levites” teaches that Hashem does not elevate a person to greatness until He first tests him. Here too, Hashem wanted to test Aharon to see if he was fitting to be the Kohen Gadol by giving his sons prominence in parshas Vayikra. If Aharon would complain about this it would show that he is arrogant and cares about his honour, and so he would not be not fitting. But if he keeps quiet and does not complain it would be clear that he has the attribute of humility, and thus fitting to be the Kohen Gadol.
And since Aharon passed the test Hashem now explained to Aharon the reason why He had not mentioned his name until now, by saying: “This is the law of the Olah (of that which goes up)” - this is the law of one who rises to greatness. He must first be tested, and therefore I did not mention your name until now in order to test you.
Full text in English can be found here.