[enobrev]

When I first got into full-time pro web development (about a decade ago), I started learning PHP. Just about every tutorial I read or book I wished I could afford had 'MySQL' next to 'PHP' as though they were one in the same.

From there forward, any time I was looking into a new host, they ALL had both PHP and MySQL. Any job I was bidding, discussion I was having in IRC or on Forums, anywhere - the two came as a matching set. I can't say that's WHY it became more popular, but I've always assumed their popularity were closely aligned.