Perhaps the reason why this is so, is: Clinton has already spoken privately with the heads of these companies (the companies owned by the oligarchs) and with their lobbyists, and she coordinates her campaign with their propaganda-operations. So: her messages are also their messages. (But what she has told them behind closed doors goes even beyond that, into her proposing new federal subsidies for their businesses.)
The historical background of the current developments in the U.S., has already been well covered in a lengthy paper by Ryan Patrick Alford, that’s appropriately-titled “The Dismantling of the Rule of Law in the United States” (especially see there “Citizens United,” which is a direct source of this).
But, whatever the reason: in the current U.S. Presidential race, there is no real contest at all, in terms of support by the oligarchs — and their support tends to be decisive. There is, amongst them, a near-100% unity around one nominee, as there has never been before in American history, at least going back to 1896, and perhaps to the founding of the republic. (Furthermore, back near the founding, there was no oligarchy; the nation started out as being, to a large extent, a democratic republic. It no longer is that.)