The ketores contained not only sweet smelling incense, but also the chelbinah, a foul smelling spice. There are Jewish souls who unfortunately are like the chelbinah. By themselves, they don't have such a good fragrance and are not something you would imagine G-d would accept, but when you mix them back in with all the other spices, they too come out like perfume.
Aharon’s mission in life was “oheiv shalom rodef shalom oheiv es habriyos,” and the part everyone forgets, “u’mekarvam laTorah.” Aharon haKohen, through his love, was able to draw these chelbinah-like souls back into Klal Yisrael and back to Torah. Therefore, explains R’ Tzadok haKohen, the mizbach haketores appears only after the initiation of Aharon into his job as Kohen Gadol. Without Aharon and those like him, without his love and outreach, there could be no ketores.