I don't think "better marketing" is the main thing going on. I think the main thing is network effects. For git specifically, I think having a giant project right out of the gate gave its initial "network" of people who knew how to use it such a huge head start that it was always going to be difficult for anything else to catch up, at least in open source, where network effects matter a lot.
I do think the story with MongoDB in particular has a lot to do with marketing, but IMO the broader story is that objects (/ "documents") are actually a more natural fit for modeling the storage of application state in oltp databases than are relational databases (though relational databases are certainly unmatched for analytical use cases). I'd say that the popularity of MongoDB despite the relative weakness of its implementation of that insight has been more annoyingly detrimental to the object/document store approach than to the relational DB approach. That is, the backlash to "MongoDB doesn't work very well" has been "just use postgresql" rather than "we need a better document database implementation".
Coincidentally, just yesterday I was looking into pg's object store capabilities in order to suggest "use pg as a hybrid relational / document store" as a potential compromise solution to get to consensus in a relational vs. document-store discussion. But I found it pretty lacking, frankly. It's nice that jsonb exists, but its capabilities seem quite limited. For instance, it seems I can't use json-schema to define the schema of a jsonb column? (I did conclude that I need to do more research before rejecting this out of hand, but my initial research was not as promising as I had hoped.)
I do think the story with MongoDB in particular has a lot to do with marketing, but IMO the broader story is that objects (/ "documents") are actually a more natural fit for modeling the storage of application state in oltp databases than are relational databases (though relational databases are certainly unmatched for analytical use cases). I'd say that the popularity of MongoDB despite the relative weakness of its implementation of that insight has been more annoyingly detrimental to the object/document store approach than to the relational DB approach. That is, the backlash to "MongoDB doesn't work very well" has been "just use postgresql" rather than "we need a better document database implementation".
Coincidentally, just yesterday I was looking into pg's object store capabilities in order to suggest "use pg as a hybrid relational / document store" as a potential compromise solution to get to consensus in a relational vs. document-store discussion. But I found it pretty lacking, frankly. It's nice that jsonb exists, but its capabilities seem quite limited. For instance, it seems I can't use json-schema to define the schema of a jsonb column? (I did conclude that I need to do more research before rejecting this out of hand, but my initial research was not as promising as I had hoped.)