Can't really know their reasons, but for me - it gets the job done. Not everything is 100% success and require scaling, not everything 100% will grow to huge codebase requiring code burecracy. You can do a lot with PHP until you really hit the wall, and if you get there - it is a good problem to have. Learning new languages and frameworks is great, but it is not required to fulfil business needs and if they have PHP experience and PHP does what they need - it means that PHP is probably a good tool for them at that time.