1. The World: Included in the guide was a map and the histories of the various regions of Tamriel. If anyone would be to see how expansive the world of Tamriel, not too mention the regions abroad, they would realize instantly that today's gaming and game making technologies would be incapable of producing a high quality game that included the details everyone expects out of the Elder Scroll Series as well as all of its areas.
2. The Lore: The Elder Scrolls series has to be one of, if not THE most lore-rich and detailed series of stories in the world, whether compared to games, books, or otherwise. To fit all of the detail of both lore, graphics, functionality, and other aspects of the game for ALL of Tamriel (and even its abroad regions included in its history), years, if not decades of developing would be necessary.
3. The Business: On the business side of the equation, this needs to make money. So, since Elder Scrolls seem to be a series of games that will continue on to earn money for many many years, I believe that its developers/publishers wouldn't dare to upset their journey now. What I believe will occur, if the ES series would be to continue to its conclusion of its history, the seperate histories of its seperate regions will be told, played, and made in its seperate games, until all or nearly all has been finished, and then an MMO will be released.
Of course noone is saying all of the history has to be told first, and if an MMO of ES was to be released before its history's conclusion their could be expansions and updates to expand and tell the rest, but until the story is near completion, this will likely not be seen, nor should it, lest it ruin a good story and business opportunity because face it TES fans, we will always buy the next TES game.
Feel free to make your own opinions or modify my own.