People use Apache for its modules, and because it's nicely integrated with Wordpress (and there's probably other similar reasons). People use nginx because it's

People use Apache for its modules, and because it's nicely integrated with Wordpress (and there's probably other similar reasons). People use nginx because it's easy to configure, and plays nice with async backends (and there's probably other similar reasons). No-one (who can't afford to do their own testing) picks a webserver purely for speed.