I have been researching this question all week and have not found a satisfactory answer. So, I turn to you.
Exodus 30:9: You shall not offer strange incense on it, or a burnt offering, or a grain offering; nor shall you pour a drink offering on it.
I can understand the prohibition for strange incense. Why not the others?
In both the Tabernacle and Temple, the altar of incense was located inside the structure at the entrance to the Holy of Holies. (Exodus 30:6) The special incense offered on it symbolized the prayers of the people. The burnt offering, grain offering, and drink offering were intended for the main altar outside. Offering them on the incense altar would have ruined it and could have damaged the other interior furnishings as well.