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 to look deeper.
Startups seem more optimized to fast iteration and quick development. It seems to me that they chose Mongo because they prioritized iteration and ease-of-development over the hard-nosed style needed to use a relational database well.
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.
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 to look deeper.
Startups seem more optimized to fast iteration and quick development. It seems to me that they chose Mongo because they prioritized iteration and ease-of-development over the hard-nosed style needed to use a relational database well.