The answer to this question is one that is often the underlying cause of advancements in architecture and construction: newer, improved materials. Greek architecture taught the Romans much about infrastructure with the design of a basic post and lintel system; that is, the use of uprights to support a crossbar of construction. However, interiors were limited.
With the development of concrete, rather than such expensive materials as marble, such as that used to build the Parthenon, Romans were able to build on a grander scale. With this more pliable and inexpensive material Roman architects and engineers could experiment with new shapes as well as having more freedom to construct larger interiors. With the use of cast concrete, Romans were better able to utilize the design of archs, vaults, and domes. The more creative use of these structures help to make areas more open and even somewhat mysterious as alcoves and shadowed areas developed from them.