Soldato
IIRc the reason that was given was that.
When Vader sacrificed himself to save Luke and kill the Emperor, in that moment, Vader ceased to exist, and he returned to being the good Anakin Skywalker. Therefore, his force ghost would appear as the good version of himself i.e young Anakin Skywalker, and not old man Darth Vader.
that is stupid, he still wouldnt have a clue who young anakin is as hes never seen him before....so he just sees some person who could be anyone as far as luke is concerned.
utter bs.