Because when you adapt from one format to another there are things that are naturally lost, some "adaptations" end up being their own thing (Mai-HiME for example).
The problem with animation is you have a limited runtime, they just had 12 episodes and you need a end point so they adapted just the election arc, both Manga and LN naturally have their own break points but its different as they can extend, end points are also difference since a LN only needs to worry about its volume page count as a manga have not only volume end point but also each chapter.
People shouldnt fault the Anime because it had this whole arc to cover with only 12 episodes to do it, as much people think this "little moments" add due to their limited runtime they have to pick and chose ... of course there are bad adaptations but its very unlikely series can get a 1:1 adaptation with unlimited runtime and even in those cases you can end up with the anime "catching up" to the manga (as series take breaks) and then we have the dreaded "padding", Anime Originals that just exist so the Manga can get further ahead so they can adapt it.