Why do you think Birthmothers, or biological mothers, hold such a low status in the community?

In The Giver, birthmothers hold such a low status in the community because they are considered unskilled laborers.
The text hints that birthmothers have as little prestige as laborers in Jonas's community. In the story, laborers perform manual labor, so their training periods are short. Birthmothers also enjoy similarly short training periods. Tragically, they are not even allowed to raise the children they birth. So, they cannot claim the accomplishment of having raised productive adults for Jonas's community. Birthmothers are also considered inferior to Nurturers, who have the important job of tending to the very young.
Consequently, many in Jonas's society harbor disdain for birthmothers. In the story, Lily's mother tells her that birthmothers generally enjoy three easy years of living. The only work birthmothers have to do during this period is to give birth. Basically, the women are expected to birth three babies in three years. At the end of three years, the women must revert to being laborers for their community. Remember that laborers perform largely manual work and are given very little honor in Jonas's community.
Thus, birthmothers enjoy the lowest of statuses in The Giver.

In chapter 3, Lily mentions that she wishes she could be a Birthmother when she grows up, and her mother instantly chastises her for making that remark. Lily's mother goes on to say that Birthmothers only get to enjoy three years of comfort and luxury before becoming Laborers for the rest of their lives. In Jonas's highly structured, organized community, couples are assigned marriages and only fifty children are born each year. The family unit is chosen and created by the government, and Birthmothers are responsible for only providing the community with fifty new children each year. Unlike typical biological mothers in modern society, Birthmothers are not praised or revered. In Jonas's community, Birthmothers are only responsible for having the babies, and Nurturers take on the role of caretakers. The Nurturers are responsible for raising, protecting, teaching, and caring for the children, while the Birthmothers are not involved in their children's lives. The Committee of Elders then distribute the infants to government-approved families, and the Birthmothers continue to work as Laborers. Overall, Birthmothers are held in such a low status in Jonas's community because they are only responsible for having children and are not responsible for raising them.