So when do we change the title to Sumire in Kyoto?
I think it's a consequence of the fact the series has been running for 200+ chapters and at the end of the day... Kiyo's just not terribly interesting. I don't mean that she's bad but she simply is what she is, dependable and placid with seemingly not a lot of desire for herself because her happiness comes from supporting and empowering others. She doesn't really seem to ever get upset, show doubt or worry, or anything; she's just always zen.
Meanwhile, Sumire's personality has been, since her first appearances romantic (in the love and non-love senses) and emotional. She has clear worries, doubts, motivation and goals which makes her much more dynamic and engaging both to read and write about.
Honestly, there's only so much mileage you can get out of an otherwise stoic character like Kiyo making bread rusks or karaage. I wouldn't mind her showing more dimensions to her personality but I also feel like that would go against her role as being everyone else's pillar. In a sense, then, it's only logical that she'd basically turn into a supporting character in her own series. The more dynamic or high strung characters like Sumire, Tsurukoma, and Riko do things and Kiyo's meals give them peace of mind.