Reddit used to be open source and the source is still on github as a read only archive.
AFAIK back then edit history was only kept briefly. Enough to roll back an accidental edit (if you have admin privileges anyway) but not far enough back to view old versions of posts.
Of course, they would have backups, and maybe the code has changed, but I wouldn't be surprised if it hasn't changed and those backups are impractical (slow/expensive) to access.
Keeping old revisions is a common practice but it's also expensive and in reddit's case totally unnecessary.