Even though Hakhel took place on Sukkos in the eight year, which is the year following Shemittah, the Torah connects its observance to the Shemittah year.
The commentators offer different explanations why Hakhel is associ-ated with Shemittah and had to take place immediately after Shemittah.
(Ibn Ezra actually interprets the verse as teaching that Hakhel was performed at the beginning of the Shemittah year)
Malbim writes that having spent a year not working the land and in-volving themselves in Torah, after Shemittah is an opportune time to be inspired by the Hakhel reading. Shem Mishmuel says that the whole Shemittah was a preparation to elevate themselves and be ready to internalise the words of the king.
Others explain that after spending a year not working the land and cele-brating the Festivals of Tishrei, Hakhel served to give strength and faith to carry them through the next 7 years working the land.