Why can't I see the lead source channel from the lead details page?

The Source field value is removed from the lead details each time the lead business unit (BU) is changed. You enter your source field value based on a BU and set ID lookup types when you create a lead.

Since multiple business units can exist, the set ID lookups are reset each time a BU is changed to reflect the values specific for the changed business unit. For more information about lead business units and set ID lookups, see the Multiple Business Units in Leads section of the Implementation Reference guide.