I think it's tied to the rise of RESTful services. RESTful APIs lead to people getting used to thinking and working in stateless architectures, and it became t

I think it's tied to the rise of RESTful services. RESTful APIs lead to people getting used to thinking and working in stateless architectures, and it became the thing to do. If your API is stateless, why make the rest stateful? And stateful APIs can be... difficult to deal with, having to handle a lot more in your HTTP client than you do with a simple REST endpoint. I also think open source has had an impact here. A lot of open source stuff is designed to have a simplistic architecture, since the users tend to be unwilling to start up 4 servers to run an application (whether it's a business that doesn't like the complexity, or a user who lacks the resources). That drives open source toward stateless services, and open source often defines what's "popular".