It's because part of the Abrahamic Covenant is that his slaves had to be circumcised.
https://www.cambridge.org/core/journals/harvard-theological-review/article/abs/forced-circumcision-and-the-shifting-role-of-gentiles-in-hasmonean-ideology/D2DF5F3C1E32C140F4475D9F837EFE7C
https://www.cambridge.org/core/journals/harvard-theological-review/article/abs/forced-circumcision-and-the-shifting-role-of-gentiles-in-hasmonean-ideology/D2DF5F3C1E32C140F4475D9F837EFE7C