The fact that people had to pay much more for the Kindle Vella installments than it would have for an equivalent ebook was an important factor in the program's demise. When I was checking out Vella, I noticed a lot of reviewers complaining about that very thing, with comments like, "I liked the story, but it cost too much."
Reviews at the time also revealed another problem--many of the participating writers weren't consistent enough or fast enough to satisfy readers. What they probably should have done was have the whole serial written before they started so that they could release smoothly, but a lot clearly didn't do that.
On the whole, I'm surprised Vella lasted as long as it did.