Same reason why most do it, to milk the story. Look at Dosanko Gal wa Namaramenkoi as a recent example. We all knew who was going to be the end girl but the author still paraded around others just keep cranking out chapters. They've been absent for the most part since the relationship between the two was rekindled.What a farce this series is. I mean, why did the author even introduce the other girls when the only girl that dense MC-kun is even conscious as a "girl" is Kuzuhana... I mean, yeah her name's on the title but F's sake man, and events make it clear the girls have no chance, just get them to kiss already and end this series already.
Or maybe author can go with the same as the "YJK" or "We never learn" manga where the author can go into different "routes" for the other girls...