Halliday & Matthiessen (2004: 122-3):
A Complement is an element within the Residue that has the potential of being Subject but is not; in other words, it is an element that has the potential for being given the interpersonally elevated status of modal responsibility — something that can be the nub of the argument. It is typically realised by a nominal group. … Any nominal group not functioning as Subject will be a Complement; and this includes nominal groups of one type which could not function as Subject as they stand, namely those with adjective as Head … There is an explanation of this ‘from above’ in terms of function in transitivity: nominal groups with adjective as Head can function in the clause only as Attributes, and the Attribute cannot be mapped onto the interpersonal rôle of Subject. This is because only participants in the clause can take modal responsibility, and the Attribute is only marginally, if at all, a participant.
That is, the reason that a Complement that is an Attribute cannot be raised to Subject is experiential, not interpersonal.