The target market for Roundcube at least (which I have made some very small contributions to) is small ISPs, and outside of HN and a few other developer enclave

The target market for Roundcube at least (which I have made some very small contributions to) is small ISPs, and outside of HN and a few other developer enclaves, nobody in the ISP market cares about Python or Ruby or whatever else is sexy today. Wordpress runs on PHP. Drupal runs on PHP. Joomla runs on PHP. Ergo, ISPs know how to build servers that run PHP. Ergo, if you're writing software that targets the ISP market, you write it in PHP.