The formula is as old as mankind, but it was massively developed and perfected in the 19th-century, when cheap romantic novels started selling really well among young women of the working classes. The MC was always a plain working-class girl (easy for the readers to identify with), her love interest was always an extremely desirable dreamboat, and her rivals were always high-class ladies: gorgeous, elegant, sophisticated, rich and snotty. Now, normally, if a handsome hunk is given the choice between a beautiful heiress and a chick who's poor and plain, odds are 99 to 1 that he will choose the former.
Only 99 to 1?
Like, one man in a hundred would choose the penniless girl?
You must be a hopeless optimist. ¯⧹_( ͡~ ͜ʖ ͡°)_⧸¯
The MC needed to have some good points to justify the fact that she was preferred to her competition. And these good points, as established by tradition, were:
- she's kind-hearted
- she's a hard worker
- she has homemaker skills
- she's a good Christian
Any normal 19th-century working-class girl could relate to these and think: "Hey, I'm like that, too!" On the other hand, the rival characters were all children of privilege and unreligious bitches... thus the fact that in the end the MC managed to win against all odds became more easy to believe.
The formula has evolved a little since that time, reflecting social changes (for example: religion isn't all that important anymore), but the basic lines remain the same. They are like an universal standard: you find them in romantic fiction all over the world. Even in yuri. So, you see, it's not surprising that Sakurako, who is kind and caring, a great cook, a hard worker and good at all household chores, wins her desired one's heart. :-)
Hehehe, my take is a bit different.
The way I see it, these two are lovers since chapter 1. When they first met, Sakurako was a baby lesbian who wasn't very clear about this sexxor thing, and Kasumi was a more fledged young lesbian who had a better understanding of how it works. Sakurako was smitten the moment she saw Kasumi: it was love at first sight, and Kasumi could tell (she's used to having that effect on people). Then, when they were buying furniture and Sakurako boldly suggested sharing a single bed, Kasumi agreed... and at that moment the deal was sealed and they both were clear on it: they were going to be, not only roommates, but also sekkusu furendo; they both wanted it, and they both were okay with the other as a partner.
Now, at this point, you could ask: why did Kasumi agree? Sakurako's reasons are evident, she was already head over heels for Kasumi, but what about Kasumi's reasons? Well, imo, Kasumi thought: "She's cute in a puppy-ish way, she seems nice, she's sunny and cheerful, and she clearly likes me. We are going to share a room for 3 years. I might as well agree to sleep with her. It will help ease tension, and prevent angst and dejection and all those tiresome things I don't want to to deal with 'cause I'm crazy lazy."
And it's in the 3 years that followed that Kasumi fell in love with Sakurako. Yes, because of all those things several commenters have mentioned already: Sakurako is a good cook, a good homemaker, a hard worker, a reliable woman with a good head on her shoulders, and, most of all, someone who understands Kasumi perfectly and can take care of all her needs, pamper her and make her happy. The formula of old romantic novels worked just peachy! Only thing is, they were already lovers, and having sex aplenty, when the falling-in-love development happened. This wasn't how they did it in those dime novels! Ah, modern times...