Why would someone perform an incomplete root canal?

Prevent leackage: Your tooth may get reinfected because there are chances that in remaining spaces dead tissue and bacteria is growing and you are living with infection in the mouth.

Hidden canals: Root canal therapy entails removal of the infected or non-vital tissue in the canal (s) of the tooth root(s) and filling the empty canals with a special material. An incomplete root canal could be that only the nerve tissue was removed and the fill will take place after symptoms subside. Another possibility is that a canal was not visualized therefore not instrumented and filled.

Inability to fill: I can't think of a reason one would intentionally leave a root canal incompletely treated unless they were unable to fully negotiate or find the canal. The purpose of rct is to reduce the bacterial load found within the tooth to a level that allows the host to repair the damage caused by the iinfection. Fully cleaning and filling the canals is the process required to fullfil this treatment.

Difficult procedure: Many root canals are complicated, general dentists are trained to do endo, but a microscopic endodontist is trained for2-3 more yrs, and much better at understanding complex root anatomy, and handling cases routinely that a gp finds difficult or impossible. Most failing root canals that i see can be handled great if they had presented originally to the specialist!

Different reasons: The tooth could be infected for a long time that caused the canals to calcify closing or narrowing those canal making it difficult to fill, or the dentist did not fill the canals to the proper length of the roots.