My guess would be the German decision to leave troops behind in French and Belgian coastal cities. After the breakout from Normandy, the German strategy was to pull back to the Rhine, but leave significant garrisons in the coastal ports. This threw a serious monkey wrench in Allied logistics. The port facilities in Normandy were not big enough to handle the logistics for a quick win.
“The port facilities in Normandy were not big enough to handle the logistics for a quick win.”
Nor close enough when the push east toward Germany was in full swing.