This is a really good question. I personally think it is extremely unfair for Itsuki not to give her even a chance. I'm waiting to see if this author's choice will be mentioned in Itsuki's character book.
I don't think it needs to explain. I agree that Itsuki should have been given more screentime to showcase her character however I think they shown clearly that Itsuki sees Fuutarou as a close friend as they have a lot in common as well as a rival somewhat.
In chapters 117 and 118 it was seen that Itsuki had some problems with herself and did not feel fully capable of congratulating Yotsuba and Fuutarou. After the various events during the two chapters, Itsuki realizes that she too has fallen for Fuutarou, but that there is no more room for her feelings now that Fuutarou has declared himself. So Itsuki decides to keep her feelings towards Fuutarou secret, trying to see them as a precious memory (Itsuki memoires, as the title of the chapter) of this period of her life.