It has a markup language that accomplishes the same thing effectively but has a completely different syntax. This is super annoying when you are bouncing between pull requests in Atlassian Bitbucket and JIRA tickets. I have tried to create a heading with # only to end up with an <ol> at least a hundred times.
I imagine this is because JIRA (2002) predates Markdown (2004), or at least its mainstream popularity when GitHub adopted it.