Wrestling is hard. It's a cardiovascular feat akin to a several mile run, only the guy you're racing also keeps punching you in the face and throwing you at the ground. It's exhausting and it hurts. That's why, generally speaking, most try to only do it once a night.
When someone does pull double duty, it's notable for its rarity and also because it almost always means a career highlight. Throughout history, a number of WWE superstars have earned their time and a half, competing twice, thrice, and sometimes even, er, quice, in a single night. Oftentimes, their fighting spirit under such gruelling circumstances has been enough to heighten the drama of their matches alone.
For the record, tournament matches and Money in the Bank cash-ins are considered valid examples of working double duty. On the other hand, there seemed little point including those times that a show-opening singles match got recycled to form a tag team main event, because that would just be a ranking of every episode of SmackDown generally managed by Teddy Long.