The reason is because everyone thinks the world is overpopulated, so adding more children is like "oh god, you are adding to the problem when you shouldn't be!" because it fits in with their apocalypse fantasy.
There's a semi legit reason for it in that "oh, well, there are tons of kids in the adoption system, why not get one of those" but such arguments fall really flat when you only expect gay people to take your unwanted adoption babies, or when you look at the fact that being adopted is often tough enough without being adopted by a family who will resent you for the fact that they were guilted into taking you instead of having their own biological kids. Better to leave such children for the people who actually DO want to adopt. Some people just have that need, and that's cool. Many people don't though, and if you would rather have a biochild, then you're not going to be a good adoptive parent, so it's better that you don't. Guilting people into it is only going to ruin some kid's life further.