Because of man's total and complete wickedness— because man refused repentance, judgment was soon to come:
The LORD said, "I will blot out man whom I have created from the face of the land, from man to animals to creeping things and to birds of the sky; for I am sorry that I have made them. (Genesis 6:7)God's judgment had not yet come on the earth. The men of the pre-flood world still had time to repent, but God, in His infinite knowledge, knew that they would not. Thus while the judgment had not yet come, that the judgment would eventually come was certain; for God states in this passage that He will blot out man.
The judgment which was soon to come would be specifically inflicted upon man, but it would also wreck havoc on the entire land, resulting in the destruction of many other creatures. All animals, creeping things, and birds of the sky would be blotted out, except for those on board the ark.
It's important to understand the meaning of the word "blot" in this passage. It means to destroy, to wipe out, to erase. It's used in several other places throughout the books written by Moses (Exodus 17:14, 32:32-33, Deuteronomy 9:14, 25:19, 29:20). Exodus 32 states (emphasis added):
Then Moses returned to the LORD, and said, "Alas, this people has committed a great sin, and they have made a god of gold for themselves. "But now, if You will, forgive their sin--and if not, please blot me out from Your book which You have written!" The LORD said to Moses, "Whoever has sinned against Me, I will blot him out of My book. (Exodus 32:31-33)The word "blot" does not refer to a partial removing; it refers to totally wiping something out. That something in Genesis 6:7 is man. God would soon wipe out the entire race of man— all of humanity. All of humanity would be blotted out by the flood.
The last part of Genesis 6:7 repeats the reason for which God chose to blot out man: for I am sorry that I have made them (for more information see The LORD Was Sorry That He Made Man On The Earth). Man had consistently chosen to ignore and despise his Creator and the judgment for his wickedness was quickly approaching.
Noah Found Favor In The Eyes Of The LORD - Genesis 6:8
The LORD Was Sorry That He Made Man On The Earth (Part 1) - Genesis 6:6
The Total Wickedness of the Pre-Flood World - Genesis 6:5
Righteous Men of the Pre-Flood World - Genesis 4:26
The Cave Men of Genesis - Genesis 4