In our parsha, HaShem addresses Moshe in a manner which is unique and different from every other parsha in the Torah. Why specifically in this parsha does HaShem command Moshe to assemble "the entire community of B'nei Israel?"

The Alsheich has already explained that since at the beginning of the parsha we are commanded to holiness ("Be Holy"), we could have thought that not everyone can attain this level, and therefore the command is not relevant to the entire community. Therefore the Torah stressed the opposite: "To the entire community of B'nei Israel… be holy." Holiness is not something attainable only by exceptional individuals, but by "the entire community of B'nei Israel."

I want to suggest another explanation, but in order to answer the question, we need to clarify  exactly what the Torah is commanding here, what it means by "being holy" and how this is connected to all the community of B'nei Israel.
 

Different interpretations have been given for this command. Rashi explains that holiness is manifested by abstinence from illicit relations "as every place where you find a barrier before promiscuity, you find holiness."
 

The Ramban interprets this as a general command which expects a man to sanctify himself by restraint regarding that which is permitted him, and not to be a self-indulgent hedonist with the sanction of the Torah.
 

A third explanation was given by Rabbi Shimon Shkop in his introduction to Sha'arei Yosher. He explains that the command "be holy" is explained by "for I am holy" – HaShem created the world in order to benefit His creations, therefore the more one does for the community, the more he is holy since in this he more resembles the Creator. In the language of Sha'ar Yosher:
 

"Therefore, in my humble opinion it appears that this mitzvah encompasses the entire foundation and root of our life's purpose: that all our work and efforts be sanctified for the good of the Klal (the entirety/community), that we not utilize any action, movement or pleasure which does not (also) include some benefit for others. And certainly, in all cases of sanctification, it is designated for a lofty purpose. And behold, when one betters his ways and always aspires that his life's work be dedicated to the Klal, then even everything he does for himself, for the health of his body and soul, is also is connected with the mitzvah of holiness, since as a result of this he will benefit the community at large as well, since by looking after his own welfare he will benefit the community which needs him."
 

And so writes Harav Kook in Shemonah K'vatzim, that the more Klali one is, the more he is great and elevated.
 

"One must continuously free himself from his private issues, which fill all his being to the point that all his thoughts are always centered on his personal fate. And this reduces him to the depth of pettiness, and as a result of this there is no end to his physical and spiritual woes. But his thoughts, will and the foundation of his ideas must be given to entireties: the entirety of everything, the entirety of the world, to man, to the entirety of Israel, to all of creation. And the stronger one's all-encompassing perception is, his happiness will increase, and so will he merit the illumination of the G-dly light."

I want to suggest an additional explanation for the command "be holy." The parsha opens with "Speak to all of Israel… be holy" – for the regular reader, it seems that holiness is exemplified by the nazir, who separates and secludes himself. But here, the command is for all, with the emphasis on the entire community of B'nei Israel.
 

I would suggest reading the parsha this way: "be holy" is the "heading" and afterwards come the details of all the day-to-day mitzvot. This is the Torah's way to be holy. If you really want to be holy, there is no need for fasts or self-induced afflictions, but rather uphold the laws and statutes of the Torah. The parsha speaks of honoring the elderly, Lashon Hara (negative speech) and loving your fellow man. From the viewpoint of Judaism, this is true holiness.
 

There is a famous story about the Chafetz Chaim who was once seen running through the streets of Radin late at night when he was very old. When he was asked what happened, he answered that he remembered that someone came to fix a leak in his roof and he hadn't paid him yet, and one must pay on time. Not to delay the wages of a worker – that is holiness.
 

Next week we will celebrate Yom HaAtzmaut. On this day, when we rejoice over the great gift we received from HaShem, we must also remember our obligations, that it is our duty to fill our State with holiness – meaning a State with Mussar (morals), with a holy population which takes care of the weak segments of society, where people live righteously, and which will be a light to the nations not only in terms of high tech and innovation but also in terms of religion and Mussar, that this holiness will be genuine holiness, and that this will be expressed by each and every one of us in our daily actions.

Perhaps this is the reason that this parsha opens with the entire community of B'nei Israel: holiness exists specifically in the entirety of the community, the Torah commands us the communal mitzvot and commands all of us together to act accordingly. And only in this way can we reach genuine holiness.