Why was there a severe Aveilus now? They already cried over Yaakov in Egypt!
Malbim: As long as the Mes was with them, it seemed to them that he was alive with them. Now that they crossed the Yarden, Egypt felt a great loss through his death, and a great deficiency that they did not merit that he be buried in their land. Then, his merit would have protected them, just like through his Berachah to Pharaoh, the days of famine were shortened.
Why did they say, "this is a severe mourning for Egypt"?
Malbim: Since they mourned over him a second time, they realized that the mourning is not due to the Mes, rather, for Egypt itself. Therefore, [Bnei Eretz Kena'an] did not join in this Aveilus, since it is not relevant to them. Just the contrary, they gained that the Tzadik will be buried among them!
Ha'amek Davar: Indeed, from this [Anshei Kena'an] understood that the Aveilus is not due to [the honor of] Yosef and Yaakov's other sons, and the others mourn for Yosef's honor. Rather, it is Egypt itself that mourned. Once the Mes left Egypt, they considered it as if he was already buried, like when they hand a Mes over to those who will carry him - Aninus stops, and Aveilus begins. "Va'Ya'as l'Aviv Evel Shiv'as Yamim" - Yosef delayed his father's bier there, for the Egyptians to eulogize there seven days, to teach that they should mourn over him like for their dead, since he benefited them so much via making the river rise. He is like "Av Hamon Goyim" (17:5) to them.
