Why did the Pilgrims think Plymouth Bay was a good place to buid their colony?

They didn't, really. It happened to be where they landed. It was not a good place for growing crops, and the weather was harsh, especially in winter. However, because the journey to America had been dangerous, and so many people had gotten very ill and died, they were simply grateful to be on land. They survived because they were aided by Native Americans, and with the arrival of more people, the colonies began to grow into large settlements.