It was already important enough to reach the king's ears (though they didn't know that yet), so at that point it was yet another good proposal by Isagi, but nothing warranting such a response.
Looking again even he seems weirded by their reaction :v, Bro better get used to it since they're his future in-laws
I think their reaction would be due to brining merchants since that's kinda of a big deal for a village in the middle of nowhere, like, it's not just a shop but a shop that is going to bring merchants from everywhere, that will bring more business and create jobs
Truth be told realistically speaking the entire village would inevitably end working for Isagi cause it's all about farming now lol, if anything as usual for manga it's downplaying just how much of a big deal is everything he's doing
Like, where do all these merchants sleep after days traveling? What he's doing would turn the village into a commercial hub or something lol