Seforno: To teach us that, despite the fact that not one animal of Yisrael died was a phenomenon that belied the imagination, ¹ Pharaoh exerted himself ² to overlook the consequences and refuse.

Hadar Zekenim: Because, after the first five Makos (i.e. beginning with the next Makah, Shechin), Pharaoh would not willingly detain Yisrael. Hashem would force him to do so, in order to demonstrate His capabilities.

Because whereas there was no way of checking regarding the previous plagues, here it was possible to check the animals to see whether any of them had died. ¹

Malbim: Because, in fact, one animal died, of a man born to a Yisra'elis from an Egyptian. The Torah considers him to be an Egyptian, but Pharaoh thought ¹ that he was a Yisrael. Therefore, he hardened his heart.