Sure anyone and their pet hamster could develop for Ghost and get what they want out of it, however, clients know Wordpress, non-technical account manager type people know Wordpress, inane managers know Wordpress, developers know Wordpress, people that draw pretty pictures in Photoshop and call it design know Wordpress and even my Dad knows Wordpress. I have not heard any of them want to 'avoid PHP' (they only know it is Wordpress in most cases), they don't complain about 'feature bloat' (they don't have the full admin login, just author rights), they are happy with WYSIWYG (and haven't a clue what Markdown is) and finally, they want something they know rather than something completely different!!!

So it is down to network effects. Everyone knows Wordpress and the famous five minute install.