As many here said, its easier to install and works right out of the box for me and many others that by itself is the number 1 reason.
I have tried to install Discourse on many different systems and I haven't been able to get it to work at all.
Also not everyone is a server admin, or a web developer or even a web designer so just trying to run discourse becomes almost an impossible task, when it requires so many things just to install it. like ruby, redis and postgres they are a must not only that but they have to be configure right and make them work on whatever machine you have. Just doing that requires for you to be almost like a server admin and that's if you have access to the server and rights to install such programs.
So why is it better, because you want a forum application that you can manage with ease and only worry about your members and content of your forum. Not have to worry about the limitations your server has and having to know what SSH is or how to configure a server or keep an eye on on other technical things.
That is why wordpress has become one of the most popular applications on 99% of servers out there because it just works without having to know 1000 other things, just unzip, follow the install wizard and you're done.
Remember this, not everyone is a server admin, or a programmer, have access to certain server configurations or the rights to install other applications or even the know how.
In my adventures around the internet I have probably stumble upon more PHPBB forums than any other forum application out there and as much as people hate it it is still the one application that people choose to install before any other, why? because its easier to install.
From there ZendForum, MyBB and simplemachines and then the rest.
As far as Discourse forums I have probably seen 3 or 4 and thats because I clicked on a few links on their showcase page aside from that none.
At the end there are probably more Flarum installs when Flarums beta was release than there are Discourse forums running.