Because it is made without salt.
Tuscan bread is not for everyone. It is low on flavor and has an unusually dense texture. So, why not just add salt to the recipe and make it more appetizing? Tradition. Legend has it that a high tax on salt back in the Middle Ages led Tuscans to making bread without it. When the tax was finally lifted, the bread remained unchanged. There are some that deny this legend and believe that it’s the rich seasoning in Tuscan food that inspire bakers to leave the salt out.
Tuscan bread isn’t widely found in the U.S., but it is still the bread of choice in Tuscany and some surrounding areas, such as Umbria. It is not really meant to be eaten on its own. It is usually served alongside dishes with a rich tomato sauce or with a very flavorful oil. It is also used in some fantastic dishes specific to this area including panzanella salad and the peasant bread soup, pappa al pomodoro.