One of the goals of Istio is to move the machinery out of the app. While a message queue/broker such as RabbitMQ or NATS is brilliant at distributing requests, you do have to build the glue — both the client and the server — into every single app, in every single language. You also need to write the edge proxy that mediates between HTTP and whatever the message queue/broker solution uses.

Istio (or rather, Envoy) acts as a plain HTTP proxy, meaning a client can just respect the standard http_proxy environment variable (which most client libraries do), meaning a client can just do HTTP and doesn't even need to know about the proxy. Even "curl" will work with Istio. And it's even simpler on the server end; all a server needs to do work is to accept HTTP connections.

Last I heard, Istio aims to go even further by supporting the proxying of any networking protocol (Layer 4).