It follows standards that you need to know if you want to be able to easily hack it.
Which means learning standards.
Ruby On Rails is really powerful, but it is not easy to start hacking on without a fair amount of base knowledge. In addition, discourse uses a lot of javascript, which makes it even more difficult to modify. The barrier to entry, in terms of creating a custom template, is quite high. You need to learn quite a few things before getting started.
On the other hand, hacking something like a Wordpress template takes almost zero knowledge. You can just start poking at things.
The lack of a relatively simple templating system is a big drawback in Discourse. Especially if you're not a rails developer, and you're running a community forum as a hobby.
I know how to program because of software such as phpbb and Wordpress, which are pretty much hacked together. Maybe because they're hacks, they have easier entry points.
I do think the Discourse people have different goals than I'm talking about here, however. If their goal was to make software that was easy to modify, hack, and deploy, they would have made different choices.