A recent contract job of mine used Apache because they had written a critical component of their application as an Apache module. 10 years ago. There isn't much incentive to change it.

When people ask these questions, do they think people are actively choosing to use the old technology? That's a huge misunderstanding. It's not an apache vs nginx decision. It's do nothing vs spend precious time on a side quest to upgrade. Opportunity cost is your answer.

And re: performance, keep in mind that very few applications are limited by the speed of their HTTP server. You first look at your application servers, networks, disks, databases. If your app is truly HTTP-bound, well you're probably not still using Apache! IOW the people who NEED to upgrade from apache for performance reason already have. For the rest, there is no incentive.