If they started adding features to Github issues pretty soon it would be as slow and hard to use as JIRA. It's been a big problem with issue tracking systems for a long time. Once you add enough features and form fields, then the "add ticket" page looks like the cockpit of an old Boeing 747 (not the new glass cockpit.) Then the product manager stops putting tickets in because he can't figure out how to do it, and the developers get afraid of putting time on tickets because you try to subtract an hour that you worked on it and somehow end up adding 5 hours, and the remotes complain that it takes 40 seconds to create a ticket, etc.
There are so many issue tracking systems out there because: THEY ALL SUCK!