D+ challenges are extra challenges for people who purchased the dlcs.  Why don't they feature that content ?
OK, some people may have bought only season 3+4 pass, but I think they are a minority. And promo challenges are accessible for everyone... Also, dunno if people who only got season 1+2 still ahve D+ challenges at the moment.   But it's sad to have almost half of the rally content used in two thirds of the community challenges...