User blog comment:Miskos3/Fairy Tail 538: When the Flame Goes Out, Review/@comment-135373-20170603084151/@comment-7418318-20170604181313

Or Obra's job could have been solely to spy on the GMG and Raven Tail was just the easiest guild to infiltrate, because Ivan was in a rush to build his anti-Fairy Tail team and probably didn't ask many questions as long as they obeyed his orders.

Usually a guild is a place of comradry and trust and on top of that there are events where the members of the participant teams need teamwork, which makes it extremely difficult to infiltrate most guild and get the infiltrator to be chosen as a member of the GMG selection.

The only real exceptions (that we know anyway) among the upper echelons with a chance to advance into the finale were Sabertooth and Raven Tail, who both treated their members as soldiers who were chosen purely on their ability and obedience to their master.