I have many many reasons I choose Silverstripe over Wordpress but two simple ones are
1- I find it so much easier to train clients. What I build is just right for them, Wordpress is all over the place and what sections are used for what make no sense. So it makes me look good and makes me more profitable.
2- There have been many times I have taken on clients who had Wordpress sites and were constantly hacked. I moved the site into Silverstripe and it was clear sailing after that.
So yes there are countless reasons I prefer Silverstripe, but when a client asks why not to use Wordpress as they often do, those are the easy to explain reasons that matter to them.