Thanks for the example, I'm aware of Notmuch but had no idea it relied on Xapian. However, back to my original point, there's simply not enough people or large projects/companies that rely on and use Xapian. I'm 99% positive that there is less than a handful of people at any of the last two Fortune 500 companies I've worked for that even knows what Notmuch is. But everyone knows Elastic, MongoDB, etc..

Again, I don't think Xapian is a bad tool or that it's underperforming by any stretch, but OP should not be surprised at all that most people have not heard of it.