Well, I got that but why?
Because open primaries let the opposition sabotage your Party's selection of candidate. When they have an unopposed primary candidate, as Gore and Boxer were in 2000, the Dem operatives and faithful can safely ignore their own primary, and use their votes to saddle Reps with unrepresentative candidates, or to water down grass movement groundswells. People like Duf welcome that, because it skews the Party leftward, where he has been trying to drag it for years.
Because open primaries let the opposition sabotage your Party's selection of candidate. When they have an unopposed primary candidate, as Gore and Boxer were in 2000, the Dem operatives and faithful can safely ignore their own primary, and use their votes to saddle Reps with unrepresentative candidates, or to water down grass movement groundswells. People like Duf welcome that, because it skews the Party leftward, where he has been trying to drag it for years.