Can't Map Campaign source from Lead to Potential

Can't Map Campaign source from Lead to Potential

I created a custom field in Leads with a type Lookup and the source module of Campaigns. I want to map fields for conversion so the lead Campaign gets transferred to the Potential Campaign Source when converting. However, on the field mapping I am not given the option to map Accounts, Contacts, and Potentials to Campaign Source.

What am I doing wrong?