Everyone would expect a guy like him to marry a rich alpha bitch, a beautiful heiress from a powerful and influential family. Yet the story always goes that he falls in love with the plain poor girl (because of her many virtues: she's kind-hearted, hard-working, etc. etc.) and ends up taking her as his wife.
That's how it happens... but it's worth mentioning that there's usually a bit more depth in the male character. In most cases, hidden underneath the appearance of perfect coolness and classiness, he has a secret side nobody (or almost nobody) knows about -- and the "plain girl" MC is the one girl who sees through him and discovers his secrets. And the reason he picks her, instead of some airheaded rich bimbo, is that she understands and accepts that side of him and makes him feel comfortable about it. He can really "be himself" around her, so to say, as she is the only one who knows how to cater to the real him and make him happy.
This is how it works for Sakurako and Kasumi, too. As BugDevil accurately explained before, Kasumi is lazy, a glutton, fastidious and picky about lots of things, and Sakurako is the one person in the world who has proven she can take all that in stride, love her, pamper her and take good care of her. It's not just Sakurako who's lucky to have Kasumi return her feelings -- Kasumi, if asked, will happily tell that she is super lucky to have a Sakurako in her life.
As fantasy stories go, this one has proved to have lasting popularity with audiences all round the world.
Yeah, and there are 5,000 Corín Tellado titles with more than 400 million books sold all over the world to prove it! 
And let's not forget mainstream shoujo manga: I do believe that 90% of the series out there follow the pattern you outlined.