User blog comment:IamJakuhoRaikoben/Chapter 452: The Prelude to the Final Battle, Review./@comment-1153699-20150915213648/@comment-7418318-20150916082045

Actually the Ten Wizard Saints are not the ten strongest wizards of Ishgar.

The title is (or at least was) given out by the Magic Council Chairman. It was awarded only to exceptionally strong Wizards, but there were also political considerations.

Just look at Laxus. In the GMG he showed that he's roughly equal to the 5th strongest Wizard Saint Jura, which means he's stronger than the "last" four. And yet they didn't make him a Wizard Saint. And neither did they make Gildarts one, despite his obvious strength... or Zero or Hades for obvious reasons.

Of course that only apllies to the Wizard Saints as a whole (or rather the last six). The 4 Gods of Ishgar were specifically stated to be the strongest mages of the continent.