Postgres has existed for more than 22 years. Their community claims it is the world’s most advanced open source database. Having worked extensively on Postgres for almost 10 years I can testify that is a true statement.
Mongo, on the other hand, emerged in the late 2000s as the pinnacle of the ‘NoSQL’ movement — developers who were thirsty for easier, more lightweight approaches to databases without the need to learn SQL. It worked, and today MongoDB is backed by a NASDAQ-listed company with $4.5b market cap and 1000+ employees. Mongo was touted as the future of databases for start-ups and full stack engineers that wish to move fast without the cruft and constraints of an orthodox SQL database like MSSQL or Oracle. Therefore when The Guardian decided to build their next generation CMS on top of this technology it was a big deal for the technology and its commercial potential.