Map Fields - can't get the field I want in drop down menu

Map Fields - can't get the field I want in drop down menu

I recently added the field "Lead Source" field to accounts and wanted to map this field with the same field in Leads and opportunities but it does not come up as an option in the drop down menu in the map fields for lead conversion page I only have none, account type, ownership and rating as choices. 

How can I fix this?