Seforno: Because it is only some of the people who will confess and do Teshuvah, such as Daniel and Ezra and a few others like them - most of them did not. ¹

Ramban #1 (citing the Ibn Ezra): The Torah inserts the Viduy here in connection with the following Pasuk, to inform us that Yisrael will ultimately do Teshuvah, as Hashem intended them to when He chastised them by bringing them (back) to the land of their enemies. ²

Ramban #2: Because, even after they confessed on their sins, ³ Hashem will bring them back to Eretz Yisrael - which they will not recapture, ⁴ and where they will suffer endlessly at the hand of oppressors and enemies. ⁵

Ramban: This is another of the many proofs that the K'lalos in Bechukosai refer to the era of the first Beis Hamikdash and Churban and the period that followed, ¹ and the K'lalos on Ki Savo, to the era of the second Beis-Hamikdash and its Churban, whose termination depends on Yisrael doing Teshuvah. ²