Maybe the occasion simply required more formality than a different venue, but Caesar may also have wanted to emphasize Rome's religious hegemony, which Herod himself had always been too happy to acknowledge. Still, why Apollo's Temple, of all places? Was it simply the site's proximity to the Trans-Tiber district where most Roman Jews lived? Or was Augustus subtly delivering a message? If so, what was that message? I have no idea.
A new book just out from Cambridge by Ovid Scholar John F. Miller is entitled Apollo, Augustus, and the Poets. From the publisher's description:
Apollo’s importance in the religion of the Roman state was markedly heightened by the emperor Augustus, who claimed a special affiliation with the god. Contemporary poets variously responded to this appropriation of Phoebus Apollo, both participating in the construction of an imperial symbolism and resisting that ideological project. This book offers a synoptic study of ‘Augustan’ Apollo in Augustan poetry...The book's index does cite Josephus on the 4 BC hearing
• The only comprehensive treatment of the reflections by Augustan poets on Apollo as an imperial icon • Discusses the presentation of Apollo and Augustus by all five major Augustan poets as well as minor poets • Carefully situates the literature about Augustan Apollo within the broader culture, as known from numismatic, epigraphical, artistic, and archaeological evidence