Strangely, it is not mentioned until the end of the parashah, as if it was to be left as long as possible.
There may be a symbolism here, reflected in a Talmudic comment in Tractate Tamid that as far away as Jericho people could smell the incense that was burnt in Jerusalem in the Temple.
Smells are potent and penetrate long distances so the longer the altar of incense was left the less the impact on people’s lives.
However, Rabbi Moshe Feinstein turns what seems like a negative into a positive. A really great leader, he says, is able to send the aroma of the Torah far afield. His (we can add, “her”) influence not only permeates geographically but historically through the generations.
The altar of incense represents the original outreach projects of Judaism. So why wait so long to build this altar? Because it is the culmination of the whole sanctuary project.
Everything else has to be in place before the Tabernacle can really exert a spiritual influence.