Rashi: To teach us that, even though Par'oh had promised to send them out, he did not keep his word.

Rashbam and Seforno: Like he did regarding the frogs/crocodiles in Pasuk 11 (where he needed to harden his heart on account of the crocodiles that remained in the Nile (Refer to 8:11:2:1), and here because Hashem merely removed the wild beasts and could easily bring them back ? Seforno).

R. Bachye: Because, as opposed to the previous three Makos, where the Chartumim had encouraged him to harden his heart, by Arov he hardened his heart even though there was nobody to encourage him ? since he no longer cummoned them.

Oznayim la'Torah: To teach us that even though Par'oh's heart had softened, and he had all but capitulated when he declared "Anochi Ashalach eschem" (Pasuk 24) - he now hardened his heart and retracted.