SQL is the bread and butter of the data storage world, since it is compatible with more DBMS systems than anything else.
For every mongoDB deployment out there, there will be thousands of Oracle/MSSQL/MySQL/etc databases running critical business functions working with both off the shelf and custom software in everything from family businesses to FTSE 100s.
Besides, SQL is not a fully turing complete language which actually makes it easier to reason about and to evaluate.
Not to mention the amount of study it has had (COD NF etc) over the years.
You can do a lot with a Database by understanding a subset of syntax; SELECT , UPDATE , INSERT, DELETE , INNER JOIN , OUTER (LEFT|RIGHT) JOIN.