And quite few are using those, and for that can be found several reasons. The biggest difference between WordPress and Discourse is size, in every meaning of that.
Discourse is a forum, and WordPress is almost everything else than a forum. And that is the main reason why out there is really much bigger variety of themes and plugins in the WordPress world, and some can even make few buck selling those.
Let’s imagine 1 % of admin’ish can code so much they will produce something usefull, or something anyway. That means out there is more wordpress-coders/devs only than discourse-admins totally.
WordPress is made for almost everyone, even for those who has limited tech skills — and if they are really limited, out there is one-click installs, documents and webinars AND paid consultants to help. Discourse is a way more demanding.
Business model of Automattic (nowadays) and Discourse are totally different too.
All those together means fewer theme options for Discourse 
But is more or less false idea to claim that lesser amount of themes is Discourse’s fault and variety comes from Automattic.