Eisenhower always said that it was because the task was to defeat Hitler as rapidly as possible, and that any actions with other aims--no matter how meritorious--were subordinated to that end. Darlan had power and authority, and Eisenhower thought that installing him as proconsul in North Africa would shave two to four weeks off the time needed to consolidate the American occupation of Morocco and Algeria.
For similar reasons, Churchill told the London-based Polish government-in-exile that they needed to make the best terms with Stalin that they could (rather than expect British and American support on border or government composition issues); Roosevelt pushed the cross-channel Normandy invasion and rejected more postwar-minded proposals for attempting to make sure American and British troops occupied more of Eastern Europe when the war ended; and lend-lease to Russia was continued at an extraordinary pace--American trucks gave mobility to the Russian army in the last year or so of the war. All things aimed at accelerating the defeat of Hitler at the price of giving nasty people better postwar opportunities.
And all, I think, highly defensible: a policy of no lend-lease to Russia because Stalin was a paranoid psychopath might well have given Poles, Czechs, Slovaks, and Hungarians a better post-WWII half century, but the price paid in terms of the extended lifespan of the Nazi Empire would surely have been too high...
Brad DeLong