Because Disqus is free and takes just a few seconds to add the snippet of JS code to your site to work whereas Discourse requires a 2GIG server at $10-$20 a month and then usually hours of configuration.
Discourse may or may not be good system but I find it sad that it has such huge requirements compared to old school php forums like phpbb, smf, vbulletin etc...
Discourse really isn't very good. If you want a more powerful forum system that doesn't need as much CPU power, get SMF or Xenforo or MyBB or something. If you want some new fancy thing, get NodeBB. Many of the sites that used to use Discourse are moving to NodeBB.
We don't see many NodeBB sites in the wild. If you want an old-schoool, traditional PHP / MySQL system, XenForo is a good choice, but there are a lot of critical security exploits, so stay up to date.
Discourse may or may not be good system but I find it sad that it has such huge requirements compared to old school php forums like phpbb, smf, vbulletin etc...