I believe that's a combination of name recognition and also Amazon advertising power. Aren't those shorts published by some Amazon imprint?
The last data we had on that from Data Guy showed Amazon Imprints expanding in market share, and from the number of ads I've seen, I can't imagine that's less true today.
I checked, and Koontz has some titles in Amazon Original Stories. They were released in mid-November, and one of them is currently #1 in the US Amazon store, as well as #8 on Amazon charts. They're selling at $1.99, have audio versions, and many reviews.
So if you're a household name and publish with an Amazon imprint, you can make a killing on novellas. I doubt that's a precedent that should influence our strategies, however.
Oh, by the way, the six books are a series.