Why did Hashem need to harden Par'oh's heart specifically now?
Ramban #1: Because, whereas by the earlier Makos, Par'oh's 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