What do you mean by "if the market wanted it"? Unless people actually stopped paying for porn on non-.xxx domains, or they payed more for porn on .xxx domains, then there is no market force encouraging porn companies to change what they're already doing.

(Rant: The personification of "the market" to the point where it is capable of "wanting" things on its own initiative is a pet peeve of mine. It's not a person. It doesn't have desires. Market economics is a descriptive theory, not a theological statement.)