The gemara says that the amount of bread that one needs to knead in order to be chayev in challa is 1/10 of an Eifa which is the shiur of an Omer. The way the Torah articulates this is by saying עריסותיכם, meaning the amount of עיסה that they were accustomed to eating in the midbar. Why is the Shiur to be Chayev in Challa dependent on the amount of Mann that they were eating in the midbar?
It seems to me that the entire concept of separating challa is to remind us that our primary sustenance comes from Hashem. By connecting the mitzvah of Challa to the Manna we are reminded that the same G-d who made the miracle of bread falling from the sky in the midbar, continues to make the miracle of wheat sprouting from the ground after they enter Israel. After yerusha v'yeshiva when they start to live non-miraculous lives, eating from the toil of their labor, they need to be reminded to separate Challa for Hashem specifically when they eat the amount that was eaten in the midbar. By connecting the dots between bread from the ground and the Mann from the heaven they are able to continuously be reminded of the miracle in the midbar, and recognize that a natural miracle of bread coming from the ground is no less a miracle. The concept is similar to the Ramban's explanation at the end of Parshas Bo where he writes that Hashem doesn't perform obvious miracles in every generation, therefore we are required to remember and invoke the miracles that were performed long ago to remind us that Hashem is the כל יכול.