Because we live in a heteronormative world where unless specified otherwise, someone is assumed to be heterosexual, which leads to the erasure, invisibility, and marginalization of gender and sexual minorities.
Hope that helps!
EDIT: @Jacob W: "I hold that we should be pointing out that homosexual men are overrepresented in the child molester category and that lesbians are overrepresented in the domestic violence category."
Sources or I'm calling bullsh*t.

