A dental implant specialist has completed multiple years of extra training and schooling in regards to dental implant surgery beyond what a general dentist has. Their experience in performing surgery is most likely multiple times that of a general dentist.
An analogy would be if you were to have a simple brain tumor removed, would you see a general medical practitioner or specialist who has done many types of brain surgeries within his/her lifetime.
It all comes down to experience and numbers.