The Newcastle United midfielder impressed in the second half of the 2017-18 campaign, and there were strong calls for the 26-year-old to be included in the squad that travels to Russia.
Southgate has denied suggestions that Shelvey's perceived hotheadedness saw the midfielder left out of the travelling party.
"We've watched a lot of games this year myself and Steve Holland. We know all of the English players in the league; their strengths and weaknesses and the players we've picked are ahead of the ones we haven't," Southgate told reporters.
"In terms of temperament: we assess someone on the way they fit into the way we want to play. Some of our finest players: [Steven] Gerrard, and others who were not ours like [Patrick] Vieira and [Roy] Keane have more red cards than most so we can't just pick players who don't get booked.
"It's not the primary factor for us. It's how their style of play fits, do they have the attributes to play in our system."
Arsenal's Jack Wilshere was also overlooked for the squad in a midfield position.