There's a post that I saw on HN a couple weeks ago[1] talking about what an AWS Lambda service would look like in Cobol and I was blown away. This is the stuff my dad used to work on when he was fresh out of college, and I'm not exactly a spring chicken (as evidenced by the fact that I used the phrase "spring chicken")!
1: https://news.ycombinator.com/item?id=25989454, link to article submitted for the lazy: https://developers.slashdot.org/comments.pl?sid=18156250&cid...
Same thing today (and a few PC here and there).
Getting access to a mainframe was and still is very expensive. That's why Google, Facebook, everyone post 2000 basically got started on commodity hardware. Because it's cheap, it's what the founders knew and it works. It's also what the top 50 alumni know, and there's no vendor lock-in.
However usually these systems didn't take off because they were "before their time". There were cloud services in the 80s - but PCs got faster and cheaper than internet speeds could keep up. Client side apps looked better than cloud apps. Similarly modern data centers, and cloud computing primitives didn't exist so reliability was more miss than hit.
Now the economics have turned and people need data shared across multiple devices. Cloud services are the defacto method of developing applications.