They use don't use it because it's just 'one of the devs characters'.
"Herp Derp, hey guyz! Look at mah awesum Noard!"
"Woww cooel, lets pewt him in teh trailer!"
No. The design was a collaborated effort. Not to mention it's canon. A Dark Elf for Morrowind, an Imperial for Oblivion (Cyrodiil), and a Nord for Skyrim. They also do it because if the Dovahkiin was a different person in every video/picture, it would break preset immersion into the game. It points too obviously as being a choice to the player. There is only one Dovahkiin per 'universe', if you will. And as I said before, what would we have to look forward to if they released screens of every race in the game? Not nearly as much.