Why did Hashem need to harden Pharaoh's heart specifically now?
Ramban #1: Because whereas by the earlier Makos, his magicians encouraged him to stand firm, thereby making it possible to harden his own heart, now he was on his own and vulnerable, and he did therefore not have the strength to refuse. 1
Ramban #2: Refer to 7:3:1:2.
Seforno: Because boils, which strike the body, are something that is particularly difficult to bear. 2