I think the short answer is the same today as it was back in those days. A person once put it to me simply: "programmers are afraid of databases." It was a perfect mixture of this unfounded fear with an advertised simple solution. Add the idea that there is no need for a DBA, encourage developers to see Mongo as a magic data storage and nothing more, and you have the perfect product for those who don't feel the need…
Maybe it's an age thing (or the product of "bootcamps"), but I don't think that's true. I'm 40, and when I was learning, you connected to and wrote SQL against the database. These days I still spend a significant amount of time writing SQL, both for ad hoc queries and for complex queries / performance optimization.
> Add the idea that there is no need for a DBA
With many cloud database offerings this is essentially true. As for things like indexes, etc... developers targeting Mongo need these as well.
> encourage developers to see Mongo as a magic data storage and nothing more
In modern frameworks, this tends to be true no matter what the data store is.