Because evolution does NOT mean better. Evolution just means that a species has changed. So there could be a series of mutations that make a species WORSE and if they cannot adapt, they die out. Which is where survival of the fittest (which is technically wrong too) comes from. You can have 10 different birds, and the ones that can adapt have babies, and the ones that are fat and can't fly, maybe have some babies, but then their fat babies might not have babies, until they all die.