The reasons are listed in the release notes ( New Minor Release: Zammad 5.3! ) - additional remarks can be found here: MySQL / MariaDB utf8mb4 support - #10 by MrGeneration
Honestly, postgresql is at least widely used like MariaDB / MySQL.
It has several advantages I’m not going into detail - but you really shouldn’t be scared of it.
If the database server being used is preventing you to switch then I’m sorry.