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.