@Susan I think the use of a computer and computing was something "those scientists" were doing over at the Pentagon in the view of most traditional politicians.
Traditional politics is based on human interaction for the most part so it would make sense that most Presidents would not be naturally inclined to use what was probably a difficult tool to use at the time.
If I am not mistaken thanks to David's piece the use of computers by the government all started during the FDR administration - as he mentions this is arguable but I buy his argument that the actual use by the government happened during this time.
One would have thought Clinton would have used the computer more, but he is from the old school form of politics, it was not until this era that a President had no choice but to use tools that are common place to daily activity. Yes, the World and the U.S. has changed greatly due to advances in technology. So it is not to say that President Obama is any more astute at using a computer than all the rest, he is just at a place in our evolution where the use of the computer is common place.
So to answer your question as to why there is no computer in the Oval office ? I think some of what I have mentioned is responsible for this, and yet I would prefer the Oval Office to remain without a computer. With breeches being what they are it is probably best.
Let's keep the Oval Office like a library (without computers) in sense where real thinking and decision making can be carried out with the kind of focus necessary to achieve success.