Just curious, as i've only ever heard it was because he was willing to put the allied coalition above just the United States military and political interest. Was there actually more to it than just this?
Currently reading Eisenhower in War and Peace by Jean Edward Smith so most of my knowledge on the subject is coming from that.
Essentially, Roosevelt wanted George Marshall to take over for Eisenhower after the North Africa Campaign. However, Marshall turned down the job leading to Eisenhower being picked.
Honestly, he had a lot of experience gained from leading the North African campaign. He was able to talk both politically with Churchill and handle both the "big" personalities of Montgomery and Patton. The success the Allies had in North Africa also put Eisenhower in line for the job.
This is the quick and dirty answer. I'll try to elaborate more when I get home.