Why Britain moved from Shackleton to the jet-powered Nimrod

Why did Britain replace the piston-powered Shackleton with the jet-powered Nimrod rather than a turboprop?

Because a suitable Comet-derived jet airframe was already available and additional orders would support that aircraft program.

The Shackleton replacement did not require Britain to develop a new turboprop maritime patrol aircraft from scratch. A jet-powered civilian airliner, the de Havilland Comet, already provided an adaptable airframe. Additional orders also benefited the civil aircraft program. The Nimrod choice therefore joined maritime requirements with an available national aircraft and industrial-policy opportunity.