ויקהל משה. Why is this parsha said in front of thep resence of the entirety of Klal Yisrael?
The Gemarah Taanis (11b) says אמר ר' ירמיה בר אבא: אין תענית ציבור בבבל, אלא תשעה באב בלבד. The Gemarah does not explain why and there are different opinions in the Rishonim ranging from a practical point Bavel did not as much rain and there was no need to create fast days for rain to halachik requirements of a nasi to establish a taanis on the tzibbur. The Minchas Chinuch (284:24) explains the basis of the law is that a taanis tzibbur requires an actual tzibbur. Only in Eretz Yisrael does Klal Yisrael attain a chalos shem tzibbur. Outside the land, we are merely a collection of individuals acting together, but not a halachic kahal. Accordingly, he writes that even in Eretz Yisrael today—after the destruction and exile—since the full kedushas ha’aretz is lacking, there is no genuine chalos tzibbur. As a result, Hallel would not be recited even for miracles occurring in Eretz Yisrael today, since Hallel on a miracle depends on a functioning tzibbur. (See more about this here.) The Tzafnas Paneach Berachot (10:11) echoes the same idea. He roots the idea in the Gemarah Horiyos (3b) grounding it in Horiyos (3b) that says only a kahal in Eretz Yisrael is defined as a kahal. Why is it only in E.Y. that there a chalos tzibbur, a kahal?
E.Y. is the place where there is direct connection to Hashem. It is that connection to the Ultimate Oneness that joine together different people together as one unit.
The Meshech Chachma Re'ah asks how in Shoftim Ch. 2 the people offered a korban on a bamah if the Mishkan in Shilo was already erected? He explains that since the Aron was not present in the Mishkan there is no issur bamos. The issur is only when the presence of the Aron is there. In other words, the Aron creates the presence of the Shechina in the Mishkan and that is the place that unites Klal Yisrael thereby forbidding private alters. If the Aron representing the Shechina is not present in the Mishkan, then there is no issur on making a bama. The Mikdash is a place where the Shechina is present and joins the Klal together. When there is a place for the klal that creates an issur on making individual, private alters. That issur depends on the Shechina being present in the Mikdash and that is represented by the Aron.
The Chinuch #487 counts a lav of shechting a Pesach on a bamah. There is a specific prohibition for the korban Pesach since the holiday of Pesach is to commemorate Klal Yisrael becoming a nation and therefore it has to be offered as a national korban, as one nation, on a public alter.
This is why the parsha of Mikdash and Shabbos is said after everyone is gathered together. These two mitzvot are mitzvot where one comes to a time and place elevated with the Presence of the Shechina and that units everyone together.