A technological puzzle

Nobody ever dies on Enterprise!

In contrast, on the original Star Trek, you could almost count on at least one Security corpse (or mysterious disappearance) before the first commercial break, leading to that well-known rule of survival:

Never wear a red shirt!

So my question is simple: with all the improvements in technology between Humanity’s first steps into the Universe (as shown in Enterprise) and the glory days of Starfleet (as shown in Star Trek), why did the life expectancy of the crew (especially those dumb enough to join Security) decrease so radically?