I believe most studies of homosexuality is carried out by homosexuals, since they are the only ones interested in why, so their research should be considered suspect.
Wanting to make it look as normal as apple pie would be only one reason for asking “why.” It used to also be of great interest among normal mental health professionals who got patients who were “gay” but unhappy with the situation and were looking for a way to kick the habit. (And in some sense it still is of interest, but no longer in the mental health mainstream.) “Why” is a combination of nature, nurture, and mysterious soul.