I wonder if they cast him while Dan Curtis was out of town (since he claims he was out of town when Barnabas was cast).
I mean, he looked the part I suppose, but he didn't have that loose cannon quality, nor IMO could he have convincingly played the vampire's servant later on.
One thing I have noticed from attending DS Festivals for over 20 years is how everyone (except perhaps Mr. Davis) reveres John Karlen as an actor- even Jonathan Frid.
I don't think there's any other actor in the DS family who is more beloved than our gregarious John, and let me tell you, when he joins Grayson, Joan, Thayer, Michael, Don and the others "up there" (and may that day not happen for years to come!), many a DS fan and costar will shed a lot of tears.
Likewise, I doubt we'd be talking about Dark Shadows today if Bert Convey had been cast as Barnabas. Frid's casting, like John Karlen's, was the necessary change and "lucky accident" that led to Dark Shadows being able to sustain and become the show we still love 40 years later.