I was thinking about that today, and to be honest there's a lot of mangaka that do that, it's like a marketing scheme, they attract you with a sweeter and more comedic couple at first, while they slowly introduce other characters in way to grab your attention, and it tends to work out, and this new couple has a more difficult time becoming a couple, but I think just like you I would love to see the first couple again, but that isn't what's selling, there's plenty of stories like that.
bro its not that deep
Hahahahaha, it's just an observation.
Yeah your right I have read many manga like that as well don't get me wrong I like when the couple has drama getting together but I also like to see what happens after they get together and to me the first couple was far more interesting, his temper and what happens when he flips that intrigues me, you know like what happen why's he like that all that
What usually happens is they don't know how long the story will be renewed. Unless they are wildly popular, most authors are okay'd for one shots, sometimes two shots.
When it gets interest and gets picked up as a series, then they need to expand on the story. Sometimes it is easier to pick other characters and show their romance than try to re-open a happy-ever-after you've just all tied up.
I want more of the first story I don't see why its so short and thier story is so long