Moshe Rabbeinu (Moses our Teacher) explained to B'nei Yisrael (the Children of Israel) that he supplicated to God that he be permitted to enter Eretz Yisrael (the Land of Israel) yet his request was denied, and that Yehoshua (Joshua) instead would lead the nation into the Land. This is how Parashat Vaetchanan, which we read this past Shabbat, commences.
What is the message being conveyed by Moshe?
Every parshah in the Torah contains a unified theme. This theme is often subsurface, requiring one to dig deep in order to detect it - but once one uncovers the theme, a new and profound understanding of the Torah becomes revealed.
What is the theme of Parshat Vaetchanan? It would appear to be that of Giluy Shechinah - the Revelation of God's Presence. This is evidenced by the centerpiece of the parshah: Matan Torah/the Revelation at Sinai, which was the most exceptional and extraordinary encounter with the Shechinah ever.
The leadup to Matan Torah and its aftertext consist of Moshe's exhortation and tochachah (reproof) about adhering to the Torah and not forsaking God, in which Moshe graphically highlights the Giluy Shechinah experiences which the nation underwent:
"Did a nation ever hear the voice of God speaking from the midst of the fire, as you did, and live? Or has a god ever performed miracles, to come and take to himself a nation from amidst nations with trials, with signs and with wonders and with war, and with a mighty hand and an outstretched arm and with great, awesome acts, as all that the Lord your God did for you in Egypt before your very eyes?
"You have been shown, to know that God is God; there is no other one except Him. From heaven did He make you hear His voice, to teach you, and on the earth did He show you His great fire, and you heard His words from the midst of the fire... Face to face did God speak with you on the mountain from the midst of the fire... And you said, 'Behold the Lord our God has shown us His glory and His greatness, and we heard His voice from the midst of the fire; this day did we see that God spoke to man, and he lived... For which flesh has ever heard the voice of the Living God speak from the midst of the fire, as we did, and survived?" (Devarim/Deuteronomy 3:33-36,4:4,5:21,23)
Moshe emphasizes B'nei Yisrael's intense and palpable encounters with the Shechinah in order to inspire them to cleave to God forever.
Moshe then teaches the Shema, exhorts B'nei Yisrael not to go astray from the path of God, and (as elucidated by Rashi) stresses the importance of the Torah as the vivifying and eternal, living Word of God that sanctifies and sustains us for perpetuity by connecting us with the Divine.
Giluy Shechinah is the underlying and unifying theme to this all.
We can thus better understand why Parshas Vaetchanan commences with Moshe Rabbeinu being denied entry to Eretz Yisrael - an event that would terminate his connection with the people he led, taught and defended for 40 years.
Unlike other nevi'im (prophets), Moshe received nevua'h (prophecy) from God as word-for-word, explicated information and instruction, rather than through dreams, visions, metaphors and hints (Rashi on Bamidbar/Numbers 12:8, from Sifri). The Rambam/Maimonides (Hil. Yesodei Ha-Torah 7:6) writes that, unlike other prophets, who experienced nevu'ah through dreams or visions, via the agency of a malach (angel), in a frightened and ecstatic state, and only when God wished to convey a prophecy to them, Moshe experienced nevu'ah as clear and direct messages, while awake and upright, in a matter-of-fact and collected state of mind - and he was able to enter into a prophetic state at will and at all times(!).
What emerges is that Moshe Rabbeinu was a full-time conduit of the Shechinah. He not only was as intimately close to the Shechinah as humanly possible, but the Shechinah and unique and clear Divine communication came to B'nei Yisrael through Moshe.
The parashah begins with Moshe explaining to the nation that he would no longer be with them - meaning that a central element of their encounter with the Shechinah would be no more. (Please see the Ba'al Ha-Turim on Devarim 3:24, who appears to be making this very point.) Hence does this narrative conclude with the appointment of Yehoshua as the next leader, for as great as he was, Yehoshua lacked the exclusive prophetic quality of Moshe.
The theme of Parshas Vaetchanan of the essentiality of experiencing the Shechinah is thus introduced and embodied by Moshe's impending departure. This is why Moshe's supplication to enter Eretz Yisrael, the denial of his request, and the assignment of leadership instead to Yehoshua commence the parashah.