I don't think this has anything to do with tech stack.

React is generally easier to use correctly than plain JavaScript, PostgreSQL is as easy to use as SQLite (and more powerful), Node is as easy to use as PHP (but has more libraries, and you can use TypeScript which makes it easier to write correct code), etc.

The only actually simplifying choice seems to be using a single VPS instead of a Kubernetes-based cluster.

It's far more likely that both the financial success and the tech stack are caused by a pragmatic attitude focusing on the business itself rather than tech details, rather than the tech stack being the cause of the success.