When MySQL first started to take off (circa 1998) it had a lower memory requirement. So did PHP.
Memory was the most precious resource at that time, and so MySQL, PHP expanded rapidly.
They are both still going strong from that initial popularity spurt.
2. .:: Novice coders do not think about DB corruption ::.
:: If you are new to coding, and have not been classically trained in a Computing Degree, it seems that people don't care for transactions or consistency.
MySQL was/is worse than Postgres on both these fronts, but the users do not care. At all. And if they do care, it may be because they heard that it was important, rather than actually feeling ill without transactions.
:: I value my users and would never want to inflict data loss on them if I could avoid it. I also know SQL well from my Computer Engineering degree and industry experience with Oracle / PSQL / DB2. So I use Postgres and have since 1996.
Most people using MySQL don't realize they could cause data loss to their customers. Even if they do, they may still use MySQL and work around failures by having adequate backup strategies that cover them even if MySQL failed at a bad point.