The BBR whitepaper gave examples of improvements on high-capacity backhaul links (e.g. a 10Gbps WAN) and lower-speed, last-mile connections to end users.

DCTCP is designed for.. data center networks. You have first party control and can ensure ECN works end to end on your equipment.

BBR is designed for the "hostile internet" where you can't rely on ECN marking, and basically tons of people are willingly and unwillingly plotting against you.. middle boxes that do policing and shaping and just plain bizarre things, routers that clear options, other worse/unfair congestion controls, extreme variation in buffer sizes, etc