Because the crazy woman had to be put down before she “liberated” any more people. Kings Landing has roughly one million people living in it, and she looked to have massacred most of them and allowed her forces to annihilate the remainder. One million civilians. Who had surrendered? Who had done nothing wrong except exist peacefully under Cersei Lannister? Dany went through the street by street, decimating everyone in her path because her victory over Cersei and Kings Landing was too easy. So when Tyrion Lannister asks Jon “do you really think your sisters will bow to [Dany]?” That was his lightbulb moment. Both Jon and Tyrion had been making excuses for all of her actions and enabling her worst impulses because they loved her and thought that she could be better than her family (particularly her father).