Whereas implementing a Key-Value Store is orders of magnitude simpler. There is plenty of room for innovation, of course - but you don’t need 1,000 people to build-on some new replication scheme or distributed backend.
If you take a simple NoSQL system then tack on things like object scheme support, JSON blob value indexes, node graph support, etc you’ll end up with an RDBMS analogue.
So the barrier-to-entry to building a NoSQL system is much lower than a RDBMS - and for many companies they might decide to build their own rather than extend an existing system - leading to more completion but not necessarily a better product.
CockroachDB is the newest from-scratch SQL engine that I am aware of: https://www.cockroachlabs.com/
They re-use the RocksDB K/V store maintained by Facebook (in C++) as the storage layer, and their own code is written in Go, so I suspect that their development work is probably a lot less time-expensive than the teams working on older SQL databases.
Based on license revenue it's actually Oracle and MS SQL server by miles. They are currently #1 and #3 on DB-Engines.com. (https://db-engines.com/en/ranking) MySQL is #2. PostgreSQL is #4, though it's pretty far back from the first three.
I mean PostgreSQL is open source, and required no license fee. I'm not sure that license revenue is a good comparison metric.
It's popular on HN to focus on OSS products, but there's a very large proprietary RDBMS market measured both in terms of revenue as well as users. That's how Oracle and MS got to be the behemoths they are today.