PHP is definitely a huge chunk of the reason why nginx has taken over Apache. php-fpm with nginx is the defacto standard, and PHP is far more prevalent than a lot of people think. Apache's mod_php(X) vs. nginx+php-fpm isn't even a debate. If someone is currently using Apache+mod_php, they probably have a smaller product that will eventually have to switch to nginx+fpm in order to scale.
While I imagine PHP is the single largest reason, other languages that support or expect the use of fastcgi are also very easy to configure with nginx, whereas I can count on one hand the number of businesses I've seen using Apache's mod_fcgid.
While I imagine PHP is the single largest reason, other languages that support or expect the use of fastcgi are also very easy to configure with nginx, whereas I can count on one hand the number of businesses I've seen using Apache's mod_fcgid.