Here you go: https://www.red-gate.com/simple-talk/opinion/opinion-pieces/...

"RM:
What was key to SQL becoming the standard language for relational databases in the mid- 1980s? Was all down to good marketing?

CJD:
In other words, why did SQL became so popular? Especially given all its faults? Well, I think this is rather a sorry story. I said earlier that there has never been a mainstream DBMS product that’s truly relational. So the obvious question is: Why not? And I think a good way for me to answer your questions here is to have a go at answering this latter question in their place, which I’ll do by means of a kind of Q&A dialog. Like this:

Q:
Why has no truly relational DBMS has ever been widely available in the marketplace?
A:
Because SQL gained a stranglehold very early on, and SQL isn’t relational.
Q:
Why does SQL have such a stranglehold?
A:
Because SQL is “the standard language for RDBMSs.”
Q:
Why did the standard endorse SQL as such and not something else-something better?
A:
Because IBM endorsed SQL originally, when it decided to build what became DB2. IBM used to be more of a force in the marketplace than it is today. One effect of that state of affairs was that-in what might be seen as a self-fulfilling prophecy-competitors (most especially Relational Software Inc., which later became Oracle Corp.) simply assumed that SQL was going to become a big deal in the marketplace, and so they jumped on the SQL bandwagon very early on, with the consequence that SQL became a kind of de facto standard anyway.
Q:
Why did DB2 support SQL?
A:
Because (a) IBM Research had running code for an SQL prototype called System R and (b) the people in IBM management who made the decision to use System R as a basis on which to build DB2 didn’t understand that there’s all the difference in the world between a running prototype and an industrial strength product. They also, in my opinion, didn’t understand software (they certainly didn’t understand programming languages). They thought they had a bird in the hand.
Q:
Why did the System R prototype support SQL?
A:
My memory might be deficient here, but it’s my recollection that the System R implementers were interested primarily in showing that a relational-or “relational”-DBMS could achieve reasonable performance (recall that “relational will never perform” was a widely held mantra at the time). They weren’t so interested in the form or quality of the user interface. In fact, some of them, at least, freely admitted that they weren’t language designers as such. I’m pretty sure they weren’t all totally committed to SQL specifically. (On the other hand, it’s true that at least one of the original SQL language designers was a key player in the System R team.)
Q:
Why didn’t “the true relational fan club” in IBM-Ted and yourself in particular-make more fuss about SQL’s deficiencies at the time, when the DB2 decision was made?
A:
We did make some fuss but not enough. The fact is, we were so relieved that IBM had finally agreed to build a relational-or would-be relational-product that we didn’t want to rock the boat too much. At the same time, I have to say too that we didn’t realize how truly awful SQL was or would turn out to be (note that it’s much worse now than it was then, though it was pretty bad right from the outset). But I’m afraid I have to agree, somewhat, with the criticism that’s implicit in the question; that is, I think I have to admit that the present mess is partly my fault."

Discussed in HN (probably posted many times): https://news.ycombinator.com/item?id=39189015