Some reasons I can think on top of my head. (Which mean this may not be actual reason for mailchimp). First is ease of finding developers. This may not be so t

Some reasons I can think on top of my head. (Which mean this may not be actual reason for mailchimp). First is ease of finding developers. This may not be so true now, but the post is from four years ago. Second, and this may be more important: when initial prototyping the original developer is most fluent in PHP, thus he/she hack away first version in PHP. Thus this make the system PHP-based. Personally, while I know the goods and awesome features of Rails, Django, etc. my most fluent language is still PHP (from using it years ago). If I happen to have some awesome idea to implement fast, I would still do it in PHP.