וַיַּקְהֵל מֹשֶׁה, Moshe gathers all the people to tell them about the construction of the משכן, the Tabernacle. This is somewhat strange, for at no other time when he conveyed a Mitzvah to them does the Torah tell us that he gathered all the people together.
Perhaps the reason was based on his prophetic vision when he foresaw that the בית המקדש or Temple in Yerushalayim will be destroyed because of שנאת חינם, unfounded hatred. Jews at that time would be disunited and dislike each other for no valid reason. Because of this needless hatred Hashem will have the בית המקדש destroyed.
To avoid this same fate befalling the Tabernacle that Moshe was about to build in the Wilderness, he gathered the people and wanted them to agree to build the משכן in unity. There was to be no hatred or bad feelings among them.
Other Mitzvot were given without fear of their being abandoned because of poor relationships between Jewish people. Hence it was not necessary to gather the people when conveying Hashem’s Mitzvot. The building of the משכן was different. Moshe wanted to avoid that the Mishkan should suffer the same fate that was going to happen centuries later with the בית המקדש. Hence he had to gather the people and emphasize to them the importance of their unity.