This is a very strange post. nginx is highly mainstream and anyone working with webservers in any capacity should have come across it a number of years ago.
You would think so, but I still see new projects started every day using Apache for something nginx would be a much better fit for. I think that mostly the admins and architects on these projects have simply never heard of nginx, or if they have, they don't really know what it does.
There are many reasons for them not to go with nginx. The biggest one is probably that it's just different. Managing it is different. Configuring it is different (regardless of any similarities). Testing is still required.
It's nothing new that a lot of people either want to be lazy or are afraid of taking risks.
Being 'better' alone isn't a good enough reason to expect everyone to switch.