1. One has a better marketing machine than the other?
2. DynamoDB is pushed in slightly more balanced ways than Mongo was, at the start Mongo was supposed to be the second coming of the (database) Messiah.
3. I don't believe DynamoDB has defaults that lose your data.
Mongo works really well for backends to test your UI against in coding bootcamps. Dead simple CRUD storage and access model. As a mock React data store, it'd be hard to find simpler.
DynamoDB is a managed service that scales far beyond Mongo and truthfully far beyond what any of us here would be seriously discussing.
With regard to complex schemas and related data, both DynamoDB and Mongo are harder to deal with once you've past a trivial size. If all you need is basic CRUD with limited or no joins, Mongo could suffice to a large deployment, but if you're mostly accessing by primary key, DynamoDB will soundly eat its lunch performance-wise and for cheaper.
If you need to join related data (or just find them convenient), relational models work famously well, and usually up the point where you have 10 million simultaneous users.
Mongo more and more is being relegated to niches where the problem just happens to exactly fit Mongo's feature list, which unfortunately for Mongo encompasses ever-narrowing gaps in the other options' offerings.
Mongo can't match the scale or economy of DynamoDB and can't match the flexibility of relational. That's why fewer and fewer treat it as their go-to database nowadays.
Dynamo seems nice for some very specific use cases, but there are others where it falls short.
There's limits on how big your data can be, which is a little annoying if you want to use it to store a couple larger things alongside all your small data.
The C# API is absolutely painful IMO, and makes it easy for new developers to use it in a way that you would have been better off grabbing a different technology.
I suppose one could argue S3 would be the better choice for large payloads, OTOH I'm a fan of minimizing potential points of failure so if we already are using DynamoDB I'd rather not toss in an additional S3 integration that could break or need maintenance later.
2. DynamoDB is pushed in slightly more balanced ways than Mongo was, at the start Mongo was supposed to be the second coming of the (database) Messiah.
3. I don't believe DynamoDB has defaults that lose your data.