In my experience it really comes down to NGINX is trying to position themselves as a replacement for load balancers like F5's BigIPs. So they have functionality that goes well beyond what you'd call an HTTP Server. For instance Layer 4 proxying rather than just Layer 7 proxying.

But that's just my experience at work. I'm sure there's a ton of other reasons. I can't say that I really agree with your list. I actually find Apache's documentation to be much better, I find myself needing to go read the code to figure out some behaviors with NGINX quite often still. Process isolation with CGI can be equally done with Apache httpd as with NGINX. I can't say I'm familiar with NGINX's caching behavior in detail since we're not using it. Also not sure what you mean by vhosting ecosystem since I see the design as pretty similar, unless you mean the upstream configuration.