International surrogacy is a booming market, not just because it is legal in the United States (while it often is not in many other countries, as discussed in a previous post), but because of the legal structure of the United States, which provides more stability and freedom than many other countries.
Compared to many nations, the United States is relatively modern in the field of surrogacy. The social mores in many countries prohibit the splitting of the biological mother and the surrogate, finding that a surrogate relationship is bad for the well-being for the child.