Wallace has a foot injury. I assume Letiu is needed for the Crusaders as their third hooker.
Pahulu and Guerin are still at school. They never select players that young for these teams.
As to the Chiefs, they have never listed who is unavailable due to injury. Vakasiuola and Sa are big loses. Both have NPC experience and along with Vaenuku and Mathis would be certainties to make the NZ U20s. They still can, depending on injury and sevens commitments. Taumateine must also have picked up an injury as he was a late withdrawal from the BOP Wasps game and wasn't involved in the Blues game.