The real reason: Cheaper!
Most take a tank chassis and sacrifice the turret, leaving that off in favour of a simple casemate and fixed forward gun. They don't have a lot of side armour either, mostly just on the front end. A casemate instead of turret makes them a good deal quicker to build (and cheaper, too). This way you get more moving, armoured guns for your budget - although they are vulnerable to being out-maneuvered by tanks in close battle.
Yanks, being if nothing else adventurous, tried a different tack. They purpose built new vehicles with full turrets and went light on the armour; mobility, they figured, would be the key. High velocity gun, forget about high explosive shells, keep the front glacis stronger by leaving out the hull machine gun, that kind of thing. Sadly for the yanks, their entire doctrine regarding armoured warfare was to be found wanting in Normandy, and compensated for with numbers.
These are more or less extinct today. Various reasons for that, but a big one is they're supersceded by helicopters... and the first world is on a "peace" or "interwar" footing, so we aren't designing things around build-as-fast-as-you-can. Could change one day but i kinda doubt it; TDs are really history now.