TL;DR: they had to choose a thumbnail and summary. I have been using WikiWand for years, which not only did that but also makes reading Wikipedia much better. Maybe I'm the only one but I have the hardest time reading paragraphs with lots of words per line. Previously I had to resize the browser each time I had to read something in Wikipedia :|
I've been using it about a year now, and it has radically improved my experience reading Wikipedia.
As you point out, it's had a smoothly working link preview feature all along, as well as a beautiful, usable reading experience. Not flashy or over-designed, just clean and elegant. I get a little caught off-guard now when I see a Wikipedia page without it.
