Why would a doctor have a single chamber pacemaker with right atrium lead in someone with symptomatic bradycardia and have it vvi ?

Your question is not clear. The lead of a single chamber pacemaker can be in the right ventricle (more common) OR in the right atrium. Single lead in the atria can be used in the patient with isolated sinus node dysfunction and normal atrioventricular conduction but this approach is rarely done in the US; however, it can treat bradycardia due to problem with the sinus node. Take care.