Holy cow, those text messages.
I'm pages behind so I'm sure that someone else has opined as to why they may not have called 911. I can totally see why young adults like them might not have called 911: they were drunk or otherwise impaired so didn't want to "out" themselves or just didn't think of it as an option, college coeds from a "party" house might not be believed or taken seriously by 911, possibly previous bad interactions with LE, their brains protecting them from really believing that 911 needs to be called/the hope that the daylight hours would make everything seem "normal" again & that the roommates would start answering their texts & have some kind of explanation for the weirdness of the overnight hours, etc. Also if they would have called, they would have been asked what was wrong, perhaps asked to go look more so they could provide more info & I don't think DM or BF wanted to leave the safety of their room. Perhaps factors like being on the spectrum played a part (100% supposition on my part), which is not something we do know nor have the right to know.
I think that in times of extreme emergency, there are plenty of people who don't think to call 911. They freeze (so don't call 911 for that reason) or flee (which DM did to BF's room). Sometimes fight. But 911 is not everyone's first or even second reaction, imo.
I think they were so totally hoping there was some explanation (that they couldn't see at the time) that they waited & waited to hear from their other roommates.
Absolutely heartbreaking.
MOO.