If Planescape is open to licence from 11/02/2013 (date of that article), why hasn't anyone pick it up ? It's 2017.
It's popular by itself, but in a very small niche.
Yup. I guess the popularity of the Planescape setting is seen in too rosy terms on this board because of the admiration for Planescape Torment. Many players didn't like the setting because it was too weird. Then there was the Planescape cant, which many people hated. It also turned out, when WotC looked at the TSR financials, that much Planescape material was costly to produce and yielded little return. This was the major reason why material for the Planes was very scarce in 3.x times.
PS:T definitely would've helped Planescape books, but it came out after it had already been abandoned. The last Planescape book came out in early 1998, while PS:T was released in December 1999.
You're right that most people though it was too weird, though. Planescape as a whole is not very appealing to more casual players.
"Today we're going to run a Planescape adventure."
"Ok, DM, but what is Planetscape?"
"
Planescape."
"What is that?"
"
"
Meanwhile they could just go ahead and play this:
People still love Planescape because it was fantastic if you were willing to put in the effort of learning it. However, that was a hard sell to the mainstream audience, specially with Forgotten Realms, Dragonlance, Birthright, Ravenloft, Dark Sun, etc having a lot of great, more accessible books at the time.