Oleg,
I think the reason may be that most people feel that art needs to be "neutral", calming and inoffensive.
For business uses, firearms and nudes are verbotten, unless you own a gun related store or someplace that deals in erotica and prohibits minors. Still lifes, flowers and animals are acceptable as they don't offend anyone.
For home use, nudes aren't well accepted in america. I don't know if it's prudish or religious background, or the insecurity of our wives and girlfriends, or a prudish moral upbringing that tells us they shouldn't be in a "family" atmosphere. What I do know is the grief I (as an example) would get over putting up even the first image of your Human Details gallery from my wife, mother in law, mother and the mothers of my daughters friends. I also know that my daughters friends wouldn't be allowed over my house by thier parents any more. The thing is, none of them would object to the photos as art, they would just object to displaying it in a home.
As for the firearm photos, they run into some of the same issues. Out in the common areas of a home, parents don't want to run the risk of ailienating / offending parents of thier childrens friends. In the bedrooms, they just don't "fit". In a den, a still life of a hunting scene, or picture of a favorite animal breed seems more appropriate.
On a T-shirt or sweatshirt however, the gun pictures seem to be a way we firearms enthusiasts (gun nuts) can advertise that we support RKBA. Some of the ... ahem ....more aggresive photos you had printed on T's in the past were some of my favorites to wear.