According to a questions and answers session with the U.S. Embassy in London and AILA (American Immigration Lawyers Association), orphans are not eligible for B visas because it is likely that they are intending immigrants.
Instead, according to the February 22, 2011 questions and answers session, the adoption for an orphan should take place in the United Kingdom (U.K.). In order to do an adoption, the U.S. citizen must become a resident in the U.K. which can take up to 10 weeks. Once the adoption is complete, then the U.S. citizen can file the appropriate paperwork with USCIS and the orphan can remain in the U.K. until the paperwork is approved and completed.
This piece of information interested me, because the humanitarian in me would want the U.S. Embassy to issue the B visa to the orphan to allow him or her to come to the U.S. while waiting for the green card processing to continue. Unfortunately, the U.S. Embassy in London is taking the more strict approach, and under a strict application of B (visitor) visa laws, they are correct that an orphan has few ties to the foreign country and probably would become an immigrant to the U.S. once adopted.