Cloak and Dagger are explicitly not a couple in most of the older comics because a lot of writers felt weird about turning their relationship into a romance. Because they both kind of need each other to function (cloak more than dagger desu) having them be in a romantic relationship is weird becasue the codepency is too strong.

How ever that alos makes every single attempt at a relationship outside that pairing impossible or difficult to enjoy. How is Dagger going to enjoy domestic bliss with rando love interest if Cloak needs to come by regularly to get his fix?

Making them a couple is honestly the safest option for a good story. Especially because the rivals canon has it that Dagger is becoming a ruthless vampire slayer while Cloak is becoming more merciful to the many vampires who were turned against their will.

It hits harder for people who don't want the "will they won't they" of other couples and instead want the YA angst of 2 lovers who only have each other in a hostile world