Why might the first settlers of Jamestown have decided the peninsula was a good place to build their fort?

Perhaps because a peninsula is much easier to defend, and if you are building a fort, you are obviously thinking about safety in the event you are attacked. The peninsula allows the inhabitants of the fort to see in 290 degrees – the only encumbered view is the neck of the peninsula. So if the enemy chooses to attack from the water side, you will have lots of time to prepare, barricade yourself in the fort and get ready to do battle. If they choose to attack by land, you will have less of an advantage – but at least you know where they will be coming from. If the fort were to be set on the mainland, rather than the peninsula, the attackers could be coming from anywhere in a 360 degree radius.

Or they might just have thought it was a pretty place to build a fort.

So they could see who was approaching and perhaps travel to other parts of the peninsula by sea?