Actually, I quite agree with you. Perhaps this is just a trend, and with 26 episodes as well as the vast amount of characters, I think them living more than 10 episodes or something like that might make the series a little rushed. It's like the story's centered around Shana and Yuuji's (more of Shana, probably) encounter with the Tomogara with them as a main theme, so certain characters might simply die off without being too important later.