Hello!
There is a huge salary difference between a bams doctor and an mbbs doctor.
An mbbs doctor earns through pharmacy commission, gifts from pharmaceutical companies, commissions from laboratories for recommendations and other similar commissions, apart from the money earned by attending the patients (consultation fee). 
The consultation charge of bams doctors is less and the patient influx is also low. Plus, they don't usually recommend different tests etc. therefore, they don't earn so much through commissions.