General eisenhower was appointed supreme commander of allied forces in europe because

The answer is:

General Marshall was too valuable to remove from his position as army chief of staff

Explanation:

Franklin Roosevelt, although overruling the chief of staff on this crucial strategic issue, came to regard him as so indispensable in Washington that, when the cross-Channel assault was finally mounted in 1944, he could not let Marshall assume command of the invasion force.

There for after proving himself on the battlefields of North Africa and Italy in 1942 and 1943, Eisenhower was appointed supreme commander of Operation Overlord–the Allied invasion of northwestern Europe.

i confess that there are several parts of this constitution which i do not at present approve, but i am not sure i shall never approve them: for having lived long, i have experienced many instances of being obliged by better information or fuller consideration, to change opinions even on important subjects, which i once thought right, but found to be otherwise. it is therefore that the older i grow, the more apt i am to doubt my own judgment, and to pay more respect to the judgment of others.