Why did Google migrate from MySQL to MariaDB instead of PostgreSQL?

MariaDB is a fork of MySql so they have a lot of similarities. Also the move from MySQl is because of Oracle's handling of development and not due to a lack of features so it's only natural they move to the closest thing to MySQL. PS: This is an educated guess so take with a pinch of salt.

Note: I've spoken with several of the db team people at google on the topic of switching to Postgres over the years. I wouldn't say I can speak for them, but I think this would be an accurate representation.  They have significant in-house expertise with MySQL and the MySQL codebase, and in fact have over the years produced a number of patches to make MySQL work for their fairly unique use case. This is significant because it is likely that no out of the box database would really work well for them without some hacking, and porting their existing setup to MariaDB is worlds easier because the bottlenecks and solutions will be very similar compared to something like Postgres, and they already have a general familiarity with the MySQL internal codebase and the C++ language (both things that apply to MariaDB, but not to Postgres). This is also true on the application side, where they have written queries and code to be optimized on MySQL and deal with a number of MySQL's particular quirks. To switch to Postgres would require a significant effort to undo this code and these workarounds, only to have to rewrite queries to work with PostgreSQL's planner. And this also doesn't get into all the little nooks and crannies of operational differences, such as monitoring, where you would have to switch out all of that infrastructure to apply to a new system. Most companies don't have most of these issues, and the scale at which they operate is significantly smaller, so it makes more sense for them to switch.

Google developers contribute on mysql, so when mysql acquired by Oracle and MariaDB starts and most probably contributors are shifted. MySQL  and MariaDB are similar on later version. From its server client structure, configuration and queries. Doesn't mean PostgreSQL is more features and more matured, google will migrate into that. There a lot of thing of course to consider.