There are several reasons why the HTTP protocol is so widespread: it's the protocol used by the web; there are many tools that support working with it; and it's textual and very straightforward, which makes it easy to understand and troubleshoot even without complex tools. In Microservices architectures, it's often the primary choice for communication because it's also easy to monitor, secure, and control in distributed environments. However, it has multiple limitations that make it less efficient than other alternatives.

The most obvious downside of the HTTP protocol is that it allows only one-way communication. Although HTTP/2 allows some two-way communication, it's limited only for the purpose of requesting multiple resources more efficiently.

Another drawback is that the protocol is inherently textual. Although it's possible to send binary data over plain HTTP, the data is always wrapped in a text envelope, which means that a lot of metadata about the request and response are still in text form. Furthermore, SSE only allows us to send text data.

On the other hand, the advantage of the HTTP protocol is its simplicity and transparency. Because it's so widespread, it works well with firewalls and often works out of the box in any infrastructure. WebSockets, which don't have the limitations of the HTTP protocol, are often limited by the infrastructure. They require a separate port to create connections and often require adjustments in the infrastructure to allow us to establish a connection. Unlike HTTP, WebSocket is designed to keep long-lived TCP connections. The WebSocket-based communication consumes more resources and causes some trouble if the connections are dropped by the infrastructure policy or a failure. This is not a big issue with SSE, which expects connection failures and includes an inherent auto-reconnect mechanism.

In the end, WebSocket provides a lot more flexibility than REST or SSE endpoints