When we talk about "redshirts," we really mean security guards. Red was the color for engineering and operations as well as security, but the vast majority of the "redshirt" fatalities were security personnel. It stands to reason that security guards would be the first in harm's way and thus have the highest casualty rates. Their job is to risk -- and, if necessary, sacrifice -- their lives to protect the rest of the crew.
Right, so not only were redshirts more likely to be in potentially fatal situations (in no small part because the red contingent included security), it's also probable that there were more redshirts on board the ship overall. Yes, we saw a lot of redshirts die, and redshirt deaths were a common plot device, but looking objectively at the death rate of people wearing red shirts in TOS, it may be that the Redshirt Death Count (RDC) isn't all that alarming, given the context.