Jamieson, Fausset and Brown on Genesis 7:21

21. all flesh died ... fowl ... cattle, and ... creeping thing--It has been a uniform principle in the divine procedure, when judgments were abroad on the earth, to include every thing connected with the sinful objects of His wrath (Ge 19:25; Ex 9:6). Besides, now that the human race was reduced to one single family, it was necessary that the beasts should be proportionally diminished, otherwise by their numbers they would have acquired the ascendancy and overmastered the few that were to repeople the world. Thus goodness was mingled with severity; the Lord exercises judgment in wisdom and in wrath remembers mercy.