In an interview with Edge, game director Todd Howard explained why Bethesda prefers to develop the Elder Scrolls games in-house. Howard should know, since he's also been on the teams that created three other Elder Scrolls games: Daggerfall, Morrowind, and Oblivion.
It's not that Howard sees it as something that will never happen, since Bethesda did allow Fallout: New Vegas to be developed by a third-party. But there were two reasons why that was an exceptional circumstance. Bethesda really wanted to develop something through Obsidian, and Obsidian already had experience working with the Fallout universe, since it was founded in 2003 by many members of the team that created Fallout 1 and 2, Black Isles Studios.
As for future Skyrim intellectual properties, Bethesda will “kind of take it bit by bit” according to Howard. So maybe with enough time, if the right pitch comes along, they'll let another developer take a crack at their universe.