Generally, Servlets were multi-use objects that would serve many requests simultaneously. Except that in the early days, you could tag a Servlet as a SingleThreadModel which would tell the container to only allow one request at a time to be served by an instance of the Servlet. Lots of requests led to lots of instantiations.

This was seen as a terrible practice, whose sole benefit was to allow lazy programmers to have state variables in their controllers, and was promptly deprecated. Only to have Rails and other similar frameworks basically pick it up!