It's pretty simple, really: Destroyed nerves cells can regenerate, but destroyed myocardial cells cannot. Although heart cells do divide in younger people, the replacement is very slow and incomplete (~3%) and that process slows to a stop as we age. That's why adults with heart damage due to heart attack can't fully recover.
So translate that to an ablation. If an ablation fails, it's usually because a source of afib wasn't found and isolated, but it can also be because the burns were inadequate and didn't fully destroy the tissue, so the tissue can heal and when it does afib resumes. That's what happened to my first three ablations. When I compared the first EP's burn times with Natale's, I saw that Natale's were significantly longer.