1. He already has warned us
2. We refuse to listen
At the conclusion of the story of Lazarus and the Rich Man (Luke 16), Lazarus pleads with Abraham to go warn his brothers about the fires of hell. The story continues:
"'No, father Abraham,' he said, 'but if someone from the dead goes to them, they will repent.'
"He said to him, 'If they do not listen to Moses and the Prophets, they will not be convinced even if someone rises from the dead.'"
The implication here (listing 'Moses and the Prophets') is that people had been warned via the Scriptures of the wrath that is to come.
Jesus was likewise asked for signs, and was rather insistent back in Luke 11, when he said:
"This is a wicked generation. It asks for a sign, but none will be given it except the sign of Jonah. For as Jonah was a sign to the Ninevites, so also will the Son of Man be to this generation. The Queen of the South will rise at the judgment with the people of this generation and condemn them, for she came from the ends of the earth to listen to Solomon’s wisdom; and now something greater than Solomon is here. The men of Nineveh will stand up at the judgment with this generation and condemn it, for they repented at the preaching of Jonah; and now something greater than Jonah is here."
Again, He states, people want flashy warnings, but ignore the ones they already have.
Indeed, 2 Peter invokes the flood as well:
I want you to recall both the predictions foretold by the holy prophets and the commandment of the Lord and Savior through your apostles. Above all, understand this: In the last days blatant scoffers will come, being propelled by their own evil urges and saying, "Where is his promised return? For ever since our ancestors died, all things have continued as they were from the beginning of creation." For they deliberately suppress this fact, that by the word of God heavens existed long ago and an earth was formed out of water and by means of water. Through these things the world existing at that time was destroyed when it was deluged with water. But by the same word the present heavens and earth have been reserved for fire, by being kept for the day of judgment and destruction of the ungodly.
The point keeps being made over and over again that God has been veritably screaming from the mountaintops that wrath is coming (Read Zephaniah! or Joel! or Revelation!), but the truth is that in the last days, people say that God is asleep; that nothing will change; that people never believe wrath is coming. Whether it is the parable of the Foolish Virgins, or the Parable of the Vineyard, or the Parable of the Wheat and the Tares, Jesus is forever saying, "People get ready Judgement is coming." And yet, people ignore it.
My personal favorite - Paul writes with unassailable logic in Romans 13:
Wake up! The day of our salvation is nearer now then when we first believed!
People live in willing ignorance, as Jesus says in Matthew 24:
For just like the days of Noah were, so the coming of the Son of Man will be. For in those days before the flood, people were eating and drinking, marrying and giving in marriage, until the day Noah entered the ark. And they knew nothing until the flood came and took them all away. It will be the same at the coming of the Son of Man. Then there will be two men in the field; one will be taken and one left. There will be two women grinding grain with a mill; one will be taken and one left.
In short, yes, God did say he wouldn't flood again - but He did say He will return, and that will be a scary day. He is already shouting from the rooftops, "Repent." The question is, are people listening?