Why do US presidents serve four-year terms?

US Presidents are elected to a four year term, and can serve another. The US Constitution does not allow any more than two full terms.

Then , why did President Dwight D Eisenhower serve for 12 (twelve) years ?

Eisenhower served two full terms, a total of eight years. The only U.S. President to serve 12 years was Franklin Roosevelt, and the Constitutional amendment limiting how often a President may run for reelection was a direct result.