I forgot to mention that Mongo is much faster than SQL.
Mongo or NoSQL in general is much newer than SQL. SQL has been around for ages.
Some people don't like change. Migration is costly and hard in most cases.
Mongo is about availability of data whereas SQL is about consistency i.e: ACID (Atomicity, Consistency, Isolation & Durability)
A lot of tutorials will get programmers using SQL initially, i.e PHP tutorials