There are three main reasons for this fact.
First, there is the fact that President Roosevelt’s first hundred days were so full of activity. When Roosevelt came into office, the country was in dire need. The Depression was in full swing and something had to be done. Roosevelt came out swinging, proposing large numbers of new policies in those first hundred days. By doing so, he set the standard for focus and energy at the start of a new presidency.
The second reason is that presidents are most likely to be able to achieve things at the beginning of their term. Early in a presidency, people still have a generally positive view of the President. He (or someday she) has not yet done anything to anger people. This means that this is the best time to get things done. Presidents are evaluated after this time, then, because it should be a very productive time for them.
Finally, this is just a matter of convenience. 100 is a nice, round number. It therefore makes a good point at which to evaluate presidents.
For these reasons, it is now traditional to evaluate presidents’ performances after 100 days in office.