How do I explain to my 3-year old why his brother was adopted by us?

My bioson is 3.  At night he prays for our family and our adoptive sons mother and brother…just a simple ‘Please Bless mommy, daddy…..’.  He knows that his little brother has 2 mothers. Recently he asked me why does T have two mommies?  I was stumped…even though I thought I was prepared for this.  Our adoption was supposed to be open but it appears our sons first mom does not want to communicate with us.  Our son is is not even a year old, so I understand she may be grieving so we still send letters and photos. But….I would like some advice…from both first parents and adoptive parents on how to answer this? I don’t want to say his first mom couldn’t take care of her son, cause that is not true.  I am sure she
would have done it just fine, but she was in school and had another son that just turned 1 and she couldn’t give him the opportunities she wanted to give him.   Thoughts….advice???