Because this is a Defender L316 with a TD5 engine as used in the BMW 528i, 728i, and Z3.
(plagarised) The Td5 was the first diesel engine in the Defender to be wholly controlled by an engine management (or control) unit (EMU/ECU) - codenamed 'Thunder' and developed in conjunction with Lucas.(/)
If the EGR is giving you grief (as they do) then disconnect it, better still, remove it completely.
Or ...
Buy a real Defender (200/300tdi)