Maybe the explicitly satanic or gory cartoony stuff was no longer an attention getter. Christ and nuns and gore were eventually politically and socially safe targets. Only the newly powerless majority would protest but they had grown silent during the peak political correctness years.
Since when do old school death metal albums necessarily fulfill that criterion anyway? Most of the best death metal album art (and metal album art in general) has a cavernous graveyard feel without over-the-top cartoonish gore and blasphemy. That isn't to say that I don't like that kind of art as well but I don't think the change of style has to do with people getting bored with shock value.
The same thing is happening with movies. People are really into that computer art look right now for some reason and that's why so many movies are almost entirely CGI.
Of course it doesn't affect my opinion on a band's music but I don't think it compliments it. I have a hard time understanding how slick shiny computer-made graphics invoke a sense of morbidity. It's a "modern" look that shouldn't have any place in an underground genre that thrived in the late 80s/early 90s.
Photoshopped artwork is also pretty lame and a lot of bands who have that kind of cover art seem to do a pretty bad job at it (or maybe I just don't like the look of photoshop art).
The worst thing is when classic albums get reissues with CGI artwork. Carbonized's For the Security
has one of the worst reissue covers I've ever seen. Do labels/bands do that to try and appeal to a modern fanbase?