It is, as others have said, a backwards cause and effect. Weak candidates are more likely to have an incumbent primary forced on them. It makes more sense to run primaries every election because it will allow candidates more actual screen time.

Look at it this way. Of the last 2 elections the party that ran a primary won both times. If you do the last 20 years only Obama won election without a primary. Thats 33% odds of a no primary win.

The thing that changed? Citizen's united