Ask HN: Why do people not use multi-model DBs and go for multiple different DBs?

I see that RDBMS DBs and single-purpose DBs such as MySQL, InfluxDB (for timeseries), or Neo4J(Graph) being used and preferred by large companies or large apps to power different use cases within their Apps. At the same time, I see Redis Labs, CouchDB etc., seem to have built and offer compelling multi-model DBs that simplifies having multiple systems. But I don't see many people talking much about these multi-model DBs in general. What do these multi-model DBs miss? Do you see these multi-model DBs gain more traction in the future?