Applications and services are made up of more components and integrations today than they were just a few years ago. Coupled with a much more competitive business world, this complexity has changed everything for both development (Dev) and operations (Ops).
Dev is pressed to deliver more functionality, faster, focusing on small, incremental changes, using agile methodologies and tools such as continuous integration.
The Ops environment is much larger these days, with many more servers and services than ever before. The production environment, which used to be all physical, is now almost entirely virtualized, so that Ops can spin up servers in an instance. Ops are also managing many more resources today than ever before.