This was the last part of the city to be developed, from the 18C till today.
Before that, there were gardens and orchards with few houses. There was a large monastery just inside Porta del Popolo, which was the grand entrance to Rome before the age of trains and planes.
The first TOUR-ISTS gave this part of Rome an impetus; when they arrived from the north in their carriages (Via delle Carrozze "carriages" is here), hotels and shops sprang up to welcome them. These travelers were "Culture Vultures". It was for Ancient Rome that they came, unlike the pilgrims who had come for Christian Rome.