Deschamps likes squad players as substitudes. Not for all roles obviously, but especially in defensive positions (keepers, CB, even DM) I think he want his starters to feel confident, which is why he takes substitutes who might not be the best around, but who will train hard without pouting if they don't start. Guys like Rami or Sissoko at the WC were prime examples of that. Not sure if that applies to Lenglet though because a) I don't really know how he's like in the group and b) he actually starts quite often for France, which is baffling in a way but tbf he hasn't been crap with the NT.
Also, keep in mind that Deschamps made his list for friendlies, while the U21 list is for the Euro group stage that's starting next week. Apparently the 2 coaches talked a lot before sending this lists, so it's definitely possible that some players that would have been substitutes for the main NT were instead called with the U21 team where they'll be starters (Camavinga, Kounde, and Aouar are probably in that case, especially Camavinga who's been great with France NT, even if his current form is poor).
By the way, that France U21 team is stacked as fuck, especially on defense/midfields. Kounde, Konate, Camavinga, Aouar, Soumare, Tchouameni, Kamara... It's a bit weaker in attack (though Gouiri is damn nice), and on keepers (Lafont was promising but he's been lagging behind a bit, Meslier is a starter with Leeds but he still needs to improve a lot and makes some weird mistakes). That being said, keepers are almost always a weak links in youth team, since experience is such a big part of the role.