Humidity is Hell
Ever been to Atlanta in August? The word "stifling" doesn't quite capture the misery of triple-digit temperatures paired with humidity levels higher than a hothouse. On the flip side, North Dakota in January is on the hellish side, too, freezing living tissue in minutes and killing just about anything crazy enough to wander outdoors unprotected.
The long and short of it is that Earth's unrelenting weather would take a toll on zombies in a plethora of ways. High heat and humidity speed the deterioration of rotting flesh by providing perfect conditions for the proliferation of insects and bacteria, which decompose anything they set their enzymes to. The dry heat of a desert would suck zombies dry as husks in a matter of hours.
The bone-cracking depths of winter would cause zombie bones to be become more brittle and fragile than they already are. Even the slightest blow or stumble could make their skeletal systems completely collapse, perhaps even under their own weight.
That's not to mention deterioration caused by ultraviolet sunrays, hurricane-force winds, sheets of rain and hail, or mountains of snow. Of course, all of this foul weather may be why so many zombies prefer the relative safety of basements, dungeons and abandoned prisons.