Because in 1942, the British were more concerned
If D-Day in 1943 would've been such a striking success, why didn't the Allies do it?
Because in 1942, the British were more concerned about the Mediterranean, and the US - frustrated that the Germany first strategy, which they understood as just that and the British did not - chose to engage in multiple counteroffensives against the Japanese in the South Pacific, Southwest Pacific, and North Pacific after Midway.
There was a lot of peripheralism, essentially, which took the focus away from the most militarily and economically powerful of the Axis powers in favor of sideshows, from Italy to Burma to New Guinea to the Solomons.
Germany was the the greatest threat; the shortest road to Germany from the UK is via northwestern Europe, particularly France and Belgium.
Japan was the second greatest threat, as the only one of the three major Axis powers with a navy and merchant marine capable of threatening vital Allied interests; the quickest way to the Home Islands was via the Central Pacific.
Naturally, rather than focusing on liberating France and driving west through Micronesia to the Philippines, in 1942-43, the Allies:
a) attacked the Solomon Islands;
b) attacked northeast New Guinea;
c) attacked the Arakan;
d) attacked the French in North Africa;
e) attacked the Aleutians;
f) attacked the Italians in Sicily;
g) attacked the Germans in Italy;
h) attacked the Germans in the Aegean;
i) attacked the Japanese in Burma, again;
j) attacked western New Guinea;
Before finally, in November, 1943, began the Central Pacific drive and then, in the winter of 1943-44, began moving to actually land in France in 1944.
Eleven months after the Allies landed in France, of course, Germany surrendered.
Best,
