The big difference between a GP and an Endodontist is that the specialist has completed a 2-3 year residency at a dental school that taught them how to treat the most complicated root canals and the GP did not. All dental students learn how to do basic root canals, but when you get to weird shaped roots, multiple roots, molars endo, etc., a lot of GPs don't feel comfortable tackling them. Most GPs will do single rooted anterior root canal treatments, some will do molar endo, and they refer out the more complicated stuff.
A GP could limit their practice to only root canals, assuming that they have a strong enough patient base and that other GPs in the area will refer to them. That would be a lot easier to do in a rural area where there are no Endodontists nearby. The reason why most GPs don't limit their practices that much is because they don't have the patient flow and there are specialists nearby. I also think that most GPs enjoy the variety of a general practice, and wouldn't want to do the same thing all day.
