Because then people like me would feel the need to point out how many of the other seven deadly sins Derrick and the Duggars were committing. At the very least, they regularly commit greed, pride and sloth.
In their minds, I suspect they think issues such as abortion and homosexuality do affect a large number of people. I think they have become more outspoken about transgender issues because they see them being more widely discussed and accepted and so they feel the need to speak up and present their “Christian” viewpoint which is against any sort of acceptance.
Personally, I wish Derrick would “pounce” on the sins of child molestation and sexual assault since they affected his own wife, but that would never happen since their brand of Christianity doesn’t seem to hold men accountable for “lust” sins.