The method of conveying the Torah to Bnei Yisrael was that first Moshe heard the word of Hashem and then he transmitted it to Aharon. Then it was given to Aharon’s sons and finally to the people at large. In this week’s Portion we see a deviation from this order. The Torah starts here by saying that Moshe called all the people together at one time and he related his message directly to all the people simultaneously.
Why the difference? Rabbi Yaakov Kaminetzky gives a very interesting insightful explanation. He says that when our people were standing at Har Sinai ready to receive the Torah, we are told they were united BELEV ECHAD. They were all of one mind. They stood together and there were no disagreements among them.
When they wanted to build the Golden Calf, the Talmud Yerushalmi (San. 10;32) tells us, they were so disunited and there were numerous opinions as to what kind of idol they should build, that they finally ended up with a different idol for each tribe.
Now when Moshe was giving instructions about constructing the Mishkan and the contributions needed to build it, he had to reunite them. Hence he spoke to them all together to bring them back into the mood of LEV ECHAD, one mind. To succeed in building something holy there must be unity among the people.