Because of the greater operating angles

Outboard (wheel side) CV joints are usually the “fixed” type, also often referred to as a Rzeppa joint.  This features a bearing race with slightly arced and offset longitudinal grooves. A series of ball bearings run along these grooves. The balls are “trapped” in a cage that keeps the balls aligned to their grooves. The offset groove/caged ball design allows the joint to articulate to follow suspension travel and steering angles. These are also referred to as AC (angular contact) joints. Depending on the specific manufacturer, a typical outboard CV joint will allow a maximum articulation angle of about 47 to 50 degrees (naturally, a higher maximum angle will allow a smaller turning circle).

NOTE: Because of the greater operating angles and their greater exposure to road hazards (rocks, dirt, moisture, etc.), outboard CV joints tend to wear sooner than inboard CV joints.

http://www.autokdd.com