REST API's and microservices are a design that originated when software teams started growing in size and didn't want to lose the agility they observed when having control of their complete stack. It was enabled by a fundamental shift in infrastructure: previously, everything was managed by dedicated infra teams, but Containers and Container Orchestrators gave development teams control of their deployments. The old model of monolithic services stood in the way of developers who had access to this new power, and hence microservices were invented to address that.

I could give you similar reasons for the rise of Big Data, Infra as Code etc. Every one of these changes are driven by changes in fundamental technologies. They aren't fads.