The answer lies in the politics of coallition. The US Army and Roosevelt wanted to invade Europe directly almost from their entry into the war. Coded Roundup, there were plans for such an invasion from 1942 on.
The British and Churchill pretty much steadfastly refused to go along with such plans, instead offering alternatives that "sniped at the edges" of the German army. Churchill's Greek campaign, the North Africa campaign, Sicily, Sardinia and Corsica are all examples of this thinking.
I think that North Africa was a good idea in hindsight. It allowed the US Army to get its act together without the pain of a major defeat (Kasserine Pass was a operational one that was recoverable as opposed to a Dieppe type disaster that would have been an outright repulse with no recovery possible). Beyond that the US was probably right in that the next invasion should have been a cross-channel one. But, this wasn't in the cards because of the British and even the actual D-Day in 1944 was a hard fought political win for the US as the British still were resisting the idea and only came on board because of the pressure Stalin added to that of the US.