I work on related things at Google so have to be careful what I say here, but to clarify things a little...
In the docs it makes it clear that there are a bunch of caveats to the auto installation. Reading through these caveats, they are all there for good UX reasons, essentially to ensure that what gets advertised is what gets installed (no bait-and-switch), and that users are going to be ok with the auto-installation (doesn't eat all their data or battery life). There is advice on how to improve eligibility, it's not clear if the author followed this or not.
The eligibility factors will vary from user to user, and may between user bases as well. For example if you have an app that is popular with users on devices with less storage space, there are likely to be more who can't install it because they don't have enough space.
One or more of these factors may not have been met for the author/publisher of this app, and it's also possible that their target market also doesn't meet the conditions for auto-install as much.