As was mentioned in a past topic,Barney and Wilma generally had a friendly rapport between eachother.They seemed to like eachother,where Fred and Betty didn't seem to much like eachother.

On a few occasions where Fred and Barney had a falling out,Fred would say to Wilma"What is SHE(Betty)doing here?".Barney never treated Wilma that way.

During a few other of those occasions,Fred ordered Wilma to stay away from the Rubbles,calling them the enemies.Barney never acted like that.

The obvoius reason of course,is that Barney was a much nicer person.