It's the apparent attempt to cover up, deceive or mislead which has people justifiably upset.
As was pointed out, even if the calls to upgrade security were answered, there is still no guarantee that 4 lives would have been saved. More lives might have been lost, or maybe less; obviously a more blatant security posture would have deterred certain types of attacks. The sheer number of lost lives is not the main factor behind this story not going away. Sadly, more people die in car accidents every single day.
No disrespect intended towards those who paid the ultimate price serving their country, but realistically, if this was simply a matter of a terrorist attack which, despite the presence of the extra requested security, still claimed >4 lives, people would have dealt with it as a more straightforward tragedy, and news media would have moved on by now.
Definitely something fishy going on here, with apparent attempts to either mislead or deceive, or at the very least, obfuscate with false stories of "documentary protests" being behind things. Certainly the administration hardly went out of it's way to transparently present all that happened there.
Based on the profanity-laced vitriol in posts in other threads, and the lack of similar passion here, apparently some feel strongly this sort of dishonesty is not nearly as bad as someone incorrectly stating that Jeep might move a plant to China.
Definitely different sets of priorities there.