Tuchel announced the squad on Friday, with Palmer and Foden the most high-profile absentees alongside Trent Alexander-Arnold and Harry Maguire. England's opener against Croatia is scheduled for 17 June.
On Chelsea midfield schemer Palmer, Tuchel was direct.
"He suffers from, first of all, a lack of individual form within the club," the England manager said.
"He was not as decisive or as influential as he was in the last seasons, throughout the whole season. His record with us was just not outstanding, not good enough to make him 'no matter what, he is coming.' That is just the reality of it."
Tuchel also highlighted Palmer's repeated absences from international camps. Recurring groin and muscular problems have disrupted Palmer's season at Stamford Bridge, limiting him to 25 Premier League starts. Since Tuchel took charge of England, the 24-year-old has managed just 65 minutes on the pitch for the national team.
"He pulled out several times, injured, and when he was in camp he did not have the impact that we all wanted," Tuchel said. "I think his personality helps him in big moments, to not show nerves, and to be decisive, but to have these moments you need to be in shape, you need to be influential within this group and he has just failed to prove it on a consistent level."
The Germany head coach was notably more sympathetic when discussing Manchester City's Foden.
"With Phil it's a little bit different because Phil was excellent in camp for us. He was open, he was in good spirits. And it hurt me a lot to call him and tell him this decision."
Tuchel cited a loss of momentum in the second half of the season as decisive in Foden's case, adding that uncertainty over his best position also played a role.
"I was not even sure in the end what position his best position is. Is it a 10? Is it a false nine? Is it maybe more a Bernardo Silva role in the future as a number eight? It does not make sense to play them out of position just to have them in camp."
Jude Bellingham, Morgan Rogers and Eberechi Eze will compete for the right-half midfield role at the tournament.
Palmer scored five open-play Premier League goals this season. Whether that tally costs him further caps now is a question only form can answer.