Content filters abide by it, and it doesn't disrupt existing business.
.xxx is just an effort to make money with another new TLD. It doesn't make the web safer and it doesn't make content-filters simpler or more accurate.
Since no one is proposing forcing sites to use .xxx, those concerned with truly making the web safer should really be looking for the most effective way to get wide penetration of a labeling solution, and a meta tag is about as simple as simple gets.