Hart was the big-name absentee from the 23-man group that will travel to Russia next month, being made to pay for his disappointing form at club level.
The Manchester City keeper, who has spent the past couple of seasons on loan at Torino and West Ham United, was used heavily in qualifying and has been the Three Lions' number one in their last two tournament participations.
Southgate has instead opted for Jack Butland, Jordan Pickford and Nick Pope, however, while Fabian Delph, Danny Rose and Ashley Young have all been deemed more worthy than Bertrand for a left-back berth.
Asked to explain the decision behind those omissions, Southgate told the Football Association's official website: "Ryan and Joe have played a lot over the last two years so they're not decisions we took lightly. I could've had easier conversations by keeping them involved.
"With Joe, we've got three other goalkeepers who have had very good seasons and the decision I was faced with was do I keep Joe in and have experience around the group? Or give the three guys who have basically had a better season a chance? We felt the players all needed to be in on merit after their performances this season.
"Ryan is also very unfortunate in that it's probably one of the strongest positions we have. Ryan has had a decent season but I just felt the others were ahead of him.
"Both calls were really tough. They're both good guys and have contributed a lot throughout qualification, so it wasn't an enjoyable part of the job and I feel it's important to acknowledge their contribution in getting us to Russia."
England take on Nigeria on June 2 in the first of their World Cup warm-up matches, before facing Costa Rica five days later.