Why are garages reluctant to carry out a turbo replacement when under an insurance backed warranty?

Why are garages reluctant to carry out a turbo replacement when under an insurance backed warranty?

Is it worth replacing the blown turbo on my 2009 BMW 520d?

My 2009 BMW 520d exploded with a sudden loss of power after a brief period of a strange tiny whining noise when pressing the accelerator pedal (driving at 65-75mph). Subsequent repatriation to a franchised...