Middle boxes (https://en.m.wikipedia.org/wiki/Middlebox) are a well known source of protocol stagnation. A protocol with extensibility usually needs the client and server to upgrade, but with middle boxes there are N other devices that potentially need updating as well. Where the user (client) and service provider (server) are motivated to adopt new feature sets, the owners of middle boxes might be far less so. In net, it makes it hard for protocols to evolve.