MySQL is the PHP of databases. It was free, easy to setup and had the spotlight at the right time. The bad decisions that are baked into MySQL are plenty and really sad (like the botched utf8 defaults, the myisam storage engine, strange stuff around replication and mucu more)
It still blows my mind that they called that crappy partial buggy characterset “utf8”. Then later came out with actual utf8 and called it “utf8mb4”. Makes no sense