Fatigue. Three games going to extra-time and one day less to rest: "We could the only team to win after having played the equivalent of eight games (for the extra 90 minutes Croatia have had to play in total)".
Roberto Martinez's side won 1-0 when the two countries met in a Group G game on 28 June, but lost their semi-final 1-0 to France after knocking out favourites Brazil in the quarter-final.
"We saw in the last game that they finished the stronger of the two teams".
They also scored late in added play to come from behind and beat England in Wednesday's semi-final in Moscow.
Wembley will host several matches - including the semi-finals and final - as Euro 2020 takes place across various European cities, and Southgate said: "We nearly have a home tournament, it's going to be brilliant".
England play Belgium in the third place play-off of the 2018 World Cup at 3pm BST on Saturday, 14 July. I hope on Sunday we can celebrate together. We have always respected our opponents and we expect the same from them.
"I think in the first half we were really good and maybe we could have got another goal", Southgate told ITV.
"This may have given us extra motivation, but this is football, you have to be dignified in victory and in defeat".
"It won't be exactly the same starting XI, but we want ideally to make as few changes as possible".
"But when you've got to this point, and we've played as well as we have, you want to take those opportunities in life".
However, Southgate has a contract until 2020, which could be extended until beyond the 2022 World Cup, giving England stability.
The third-place match was dropped from the European championship by UEFA following the 1980 tournament but has remained part of the World Cup calendar despite complaints. We want to be challenging for semi-finals and finals. We have the chance to win a medal at the World Cup, which only one other English team has ever done. We are up against players of a high quality and with lots of experience.
"I'm very happy that my pupil has done better".
"I can't ask for more from the players".
Captain Harry Kane will be among the more interesting selection decisions, given the sharpshooter is gunning for the Golden Boot.
The PM's spokeswoman said: "She thinks that he led the team fantastically and that the whole country should be proud of how far England got".
But the FA have already announced that there won't be one - in fact, they aren't even letting people know when the team gets home.