And why do you think that those houses are not that popular?
I'm also interested about the opinions from others about the above question.
I also hope that one day a serie of books will appear about the lesser know Royals. I think about somekind of Rosvall book with short biographies about Royals about we don't know less or nothing. I don't share for example not the opinion of edubs31 that we know far to much about the Prussians. An example, what do we know about the daughter-in-law of The Kaiser?
And what do we know about the Badens? Wieds, Ysenburgs, Nassau-Weilburgs, Nassau-Dessaus, the Bulgarians, Saxe-Altenburgs?
I think that Queen Victoria's immediate family always has interest. The Romanovs, especially NAOTMAA, have the glamour and tragedy associated with them--not to mention the decades-long mystery first of the missing bodies and then the Anna Anderson mystery that kept it alive. Robert Massie's book helped solidify their place in the imagination of millions. Kaiser Wilhelm II is a larger-than-life figure who is so closely intertwined with the tragedy of WW1. Apart from these larger-than-life figures, and those like Marie of Romania, most royals have fallen by the wayside unless tangentially related to the above as with Vicky. There always has to be some 'hook' to draw people in and separate them from the pack.
As mentioned, the AP forum was founded to commemorate and celebrate the Romanovs. A welcome side effect of this has been the expansion of the Forum, even since I've been here, to include almost every royal family or member, no matter how obscure. This has brought a lot of attention to those royals who didn't leave some lasting mark on history but led interesting lives nonetheless.
I think a book like Royal Gatherings walks the line very well. They certainly have pictures in there of royals we know much about but along the way there are fascinating tidbits about those who may not be as widely known, even to those of us who haunt the AP Forum.
It has the requisite hook but enlightens you about others along the way. I think Romanov Autumn was so delightful and successful for the same reason. Maybe these lives weren't all as glamorous or tragic or beautiful or epic and maybe they couldn't sustain their own full-length books but there are very interesting in their own right and deserve at least some mention. You can also see Robert Golden's books here to--they contain mention of more well-known royals such as Queen Mary but mostly focus on those tangential members.
All this is just my humble opinon though.
As for the mentioned Rosvall books, I posted some about those upcoming books in the Upcoming Books for 2013 thread stickied at the top of this sub-forum.