Ridley is constantly asked, so he has to open his mouth.
Plus Ridley has a relationship with the Alien property itself. With three films under his belt including the one that started it all, Ridley has become the fatherly George Lucas of the Alien franchise of sorts. So I'm not surprised when Ridley is interviewed, that someone will bring up Alien again, again, and again.
Maybe 20th & Ridley have left open the possibility of more Alien talks down the road? I would assume severing ties with such a visionary director is the last thing they'd want to do. You don't want the experience to color other projects, even if they never do anything more with Alien. And Ridley has a relationship and ties with 20th that one would assume he wants to maintain as well. I guess we'll see. I wouldn't be surprised if 20th decides to reboot, they bounce ideas of Ridley and use him in some sort of advisory producer role.