Can you be more specific? How did it satisfy your unique business requirements for instance.

SS has saved me countless hours. When I saw that I could build a fully functional product catalogue in a few hours complete with an admin, I was hooked. Go through the tutorials, they are excellent, you will be off and running in no time. They will teach you the basics and then you will see the possibilities to build anything your client needs in a fraction of the time:
http://doc.silverstripe.org/framework/en/tutorials/

I also checked themeforest.net and there doesn't seem to be an abundant availability of silverstripe themes to choose from just yet.
It is true, but I don't think anyone spends a lot of time building 'themes' for SS, I don't think it makes a lot of sense. With SS, you can create highly customized sites and layouts but at the same type you are creating unique page types that are probably particular to your business/client. It would be hard to slam a generic template on top of that. SS is not really designed to be an out of the box solution, it's designed to let you build exactly what you want. On the other hand, once you start understanding the templating system it is not hard to take any generic template and convert it to an SS theme :0)

Silverstripe's killer feature is the combination of a good MVC structure with a well thought out & comprehensive CMS / CMF. It's really unique in the PHP world and there is very little like it in Ruby either. I'm not sure about other languages - Probably Plone or something would be the nearest competitor.

One thing that a lot of people misunderstand. The fact that functionality is defined in code is a *feature* - not a disadvantage. To be able to create exactly what you want, to have that specified in readable code that is under source control is a big advantage over setting things up through a UI & configuration for a certain level of project.

... that said, Wordpress with ACF & gravity forms with the TimberWP twig plugin is a lovely way of working for smaller less critical projects. Choose the right tool for the job.

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.