For one, it's out of reach for lots of folks who use shared hosting services, where there's a managed LAMP stack. It's rare if they have Postgres. If you use AWS, of course, everything is up to you.
There's also a connection pooling and different startup for MySQL/Mariadb vs Postgres. PHP engine generates the page and closes itself and connections. Postgres assumes you are more persistent.
Also, there's a difference in how GROUP BY works in MySQL/Mariadb vs Postgres, and wrapping your head around it, after using it the MySQL way for years, can take time. So not a drop-in replacement.