Why is the DLC for Oblivion not purchasable through Bethesda's Elder Scrolls website anymore?

Probably because people stopped buying it. If it was still being downloaded all the time, they wouldn't have removed the links.

They can still be downloaded from Steam, in the Deluxe GoTY version.

It is unfortunate that they are difficult to get now, but that's what happens when a game gets older. (it's been 7 years)