I'd say that the most important cause of the decline of oratory is a fundamental change in the political culture.
In the Roman republic, ALL political actions--proposals, debates, votes, and vetoes--were conducted face-to-face with your political adversaries, typically before the watchful eyes of the voters. If you wanted to veto a proposal in ancient Rome, you had to show up to the public reading of the bill, and in full view of everyone (including hired thugs who might crack your skull), impose your veto--and to veto the bill in a way that would be seen as immediately justifiable to everyone present. Even political actions that were not conducted before voters (i.e., senatorial debate) had to be conducted in the physical presence of a quorum of the Senate.
Today, much--maybe most--actual political action takes place in relative privacy. Bills are drafted in private, can be introduced, debated, voted up or down, and vetoed with only a sliver of the populace ever seeing the drama unfold. At least in the US, speeches can even be "read into the record" by e-mail!
Thus, the act of persuasion has moved from the public sphere toward the personal sphere, and so it shouldn't be surprising that political persuasion has lost its theatrical flavoring. Moreover, most of the little political action that is conducted in public is delivered to the public on television; consequently, political action has more of the toned-down quality of the TV actor than that of the stage actor.