There is a clear reason given.
And God said to Abraham, “As for you, you shall keep my covenant, you and your offspring after you throughout their generations. This is my covenant, which you shall keep, between me and you and your offspring after you: Every male among you shall be circumcised. You shall be circumcised in the flesh of your foreskins, and it shall be a sign of the covenant between me and you. (Gen. 17:9-11 ESV)
Circumcision is the sign of the Abrahmic covenant. The commandment to circumcise all males is reiterated in the covenant at Sinai, because everyone there was also part of the Abrahmic covenant. If a man today is part of the covenant with Abraham, he should be circumcised, otherwise it's not commanded.
The TOG