The answer is that registrars and their business partners stand to make a great deal of money from the creation of a porn-only top-level domain, so registrars will continue seeking its adoption.
How many other pure examples of profit with no communty benefit can you think of? (I suggest credit cards for students.)
Previously: Utah: No Star '80 on Port 80; .XXX will have to wait
See also: Proposal to erect XXX domain faces stiff opposition (arstechnica.com), ICANN