I think its because they don't keep up with the field. You have some dentists who learn how things are done when they are in dental school in the 1970s and that's how they continue to operate in their private practice until they retire. They might also be good friends with other specialists and will send their patients there because of that relationship (and they do at least trust their work). There are also different philosophies. Some dentists might prefer you keep original tooth if possible and have you get some advanced imaging done before they decide a treatment plan; others might lean on the side of avoiding additional imaging and just pulling the tooth assuming it is compromised.

To get around that, I prefer to get dental work done at dental schools. The faculty at the schools have their own practices within the schools, and you can be sure that they are doing all the up to date stuff that is currently being taught and following sensible practices in terms of treatment plans. Prices are also fair. If you opt for a student dentist its even cheaper and many low income people use this service (although appointments are much longer since everything they do has to be signed off by a faculty member responsible for a floor of students).