If I had to guess, I would say it’s the 20x smaller feature set compared to Elasticsearch.

We built a custom search engine on top of Elasticsearch. Our query builder regularly constructs optimised queries that would be impossible to implement in any of the touted alternatives or replacements, which almost always focus on simple full text search, because that’s everything the developers ever used ES for. There’s a mindboggingly huge number of additional features that you need for serious search engines though, and any contender will have to support at least a subset of these to deserve that title in the first place.

I’m keeping an eye on the space, but so far, I’m less than impressed with everything I’ve seen.