I generally agree with your point -- it's better to adopt orphans from one's one country.
I would guess that some people "go abroad" to adopt because the majority of prospective adoptive parents hope to get healthy children who are under two years old. So, they're willing to spend more money on an international adoption in order to avoid adopting a 10-year-old American who's in a wheelchair.
And, also, some parents hope to adopt a baby "who looks like us" -- translation, white Americans look to Russia because they're reluctant to adopt an African-American kid. (In some cases, it's not pure racism; there may be a perception, for example, that African-Amerian orphans are more likely to have been born to mothers who used drugs or alcohol during pregnancy, and therefore may have cognitive impairments and other health issues. Only, as it turns out, Fetal Alcohol Syndrome is a problem among white Russians, too...)
On the other hand, there are also some much more positive reasons for adopting abroad; it's not always about avoiding/ignoring "undesirable" orphans here in the U.S. For instance, some of the Americans who seek to adopt from Russia are themselves Soviet emigres, so they speak Russian and feel that they're in a good position to give the adopted children "pride in their Russian heritage."
Others perhaps don't know Russian, but (for example) the family has lots of prior experience in caring for "special needs" kids who have Down Syndrome or are deaf or paralyzed or whatever. So, when they hear about a disabled Russian kid who's been stuck in the orphanage system for years because most Russian parents (like most American parents) prefer to adopt a "healthy and normal" child -- again, they consider themselves to be in a better-than-average position to help this particular child.