Why, by this plague and all the subsequent plagues, did Hashem always say "Machar"?
Oznayim la'Torah: In order to give the Egyptians an opportunity to escape
(in this case by going to Goshen, where the wild animals did not appear, or to give Par'oh time to relent.
The question arises however, why He did not say Machar by Dam, Tzefarde'a and Kinim?
Oznayim la'Torah: And it seems that Chazal took their cue from here when they said in Avos 2:15 'Shuv Yom Echad Lifnei Misascha!'
Why didn't Par'oh's Chachamim not try to replicate Arov?
Riva (7:11): Par'oh did not call them, for he already saw from lice that the magicians could not do so.
Moshav Zekenim (12): The magicians saw from lice that it was all from Hashem, and their actions will not help; it is a waste of effort.