There are some reasons to explain the disparity of appeareance of the different species. One of then is taxonomy and age of these species. I mean the more blurry is the taxonomy, and the more antique the species, the more easy they have divergent looks. And those birds that inhabit in island so remote from the standpoint of occidental scientits of past times, who classified them, are more easy to be less carefully classified than our neighboury ones.
Following the same link of wikipedia it is explained that a 2009 study of " examining the mitochondrial DNA of all species to examine the relationships within the family and to its nearest relatives estimated that the family emerged 24 million years ago, older than previous estimates.". This means they have had a bit of time to differentiate among them.
" The study identified five clades within the family, and placed the split between the first clade, which contains the monogamous and , and all the other birds-of-paradise, to be 10 million years ago. "
Another reason is echological. These animals are spread on a few islands that had a very limited population of mammals and other animals that could really threaten them. Besides many of them live in very forestry areas. These two factor favoured colouring and weird feathers, because visibility is important in these areas and also they don't have to fear many predators.
And the third reason is sexual selection. For these birds the sense of the sight is essential, as it is well known.
Finally, among birds there are surprising relationships, not only in birds of paradise. To my it is atonishing that these two are related, and they are: