[acdha]

I think most of it was a reaction to the massive marketing push Mongo made. There were so many years of people hyping it as the answer to all of your data storage needs, followed by accounts of users hitting bugs or reimplementing most of a SQL database in code, and the promised performance benefits either didn't materialize or hadn't been necessary in the first place (“web scale” turned out to mean only dozens to hundreds of requests per second for many apps).

Meanwhile, Postgres was quietly plugging away adding new features and continuing to deliver solid performance for a wider range of workloads, including better performance on JSON document storage.