In a world where nanites can heal virtually any known disease and mend all but the most lethal injuries, traditional medical practice is essentially a lost art.

Changes to the medical nanites is done by analytical programs discovering necessary changes and software engineers implementing the relevant changes. When a broken bone/tendon/etc can be repaired in a matter of minutes/hours/days through use of nanites, the skills of a doctor related to such are obsolete. For diagnosis, machines are almost certainly better at picking up signs than any human would be. Again, obsolete for humans to do.

That doesn't leave much work for doctors, which is why we consider it a lost art. 

This of course causes trouble in the very rare situations that don't get picked up by the machines or won't be cured normally, but if such cases make up about 0.001% of all cases, it would be well within acceptable margins in most societies.