One led to another.
Failing to surround the Germans at Falaise led to the escape of a good chunk of that region's commanders and supporting staff. The reorganization of the German western armies combined with Eisenhower's allocation of supplies to Market-Garden led to a general stalemate, especially after Market-Garden's failure. The slow advance of the allies allowed Hitler to plan and build up forces for the Ardennes Offensive.
Now tell us what a f**k up Lee was at Gettysburg or Grant was at Cold Harbor. And how about that idiot Napoleon at Waterloo -- What an idiot!