Kylo Ren has some grandfather issues, as we saw in Star Wars: The Force Awakens. He seems pretty hellbent on procuring parts of Darth Vader to get closer to his idol, which is a little… weird.
It seems this obsession may carry on into Episode VIII too. Making Star Wars are reporting on some of Kylo's outfit choices in the next film, and they seem rooted in the past. They say his 'under-robe' is much like Luke's black outfit in Return of the Jedi, but the topper is that Ren will will also have Darth Vader's cape too, minus the chain clasp. The outlet does say they are not sure if this is a replica or if he got his hand on the real deal, much like he did Darth Vader's helmet in The Force Awakens, so there is some mystery there.
None of this is confirmed of course, but this would all make sense. The lad has suffered somewhat of a loss at the end of the first film, so I can imagine he will be doubling down on his persona in a big way. Going full Darth Vader makes sense to me.