They are different characters. In one scenario you get to see and interact with Darth Vader. In the other, you are interacting with a derivative character that is essentially culturally irrelevant. I understand you and some people you know perhaps can't tell them apart but many (though I agree not all) other people can clearly tell them apart and have an entirely different emotional connection, for some (who can afford a $10,000 vacation) going back 50 years.
So if Disney has the rights to both, why not use the one that's more culturally resonant? Are you trying to save Disney a few thousand dollars making a Darth Vader costume?
I'll be interested in checking in on the merch in SWGE this summer to see how much we see Darth Vader on merch versus Kylo Ren. Because while we can debate this intellectually, Disney has the data and they know who is more resonant, and they will follow the money.