Because there's no need to use it as soon as you get that 3rd Orb. The DoT component only lasts for 6 seconds, so you're never going to generate Shadow Orbs quick enough to keep a 100% uptime on it. Therefore, if a strong spell like Halo or Mindbender are off cooldown, you're best off using one of those instead of Devouring Plague, as long as it doesn't slow down your Orb generation, ie, delaying MB or SW
.
That said, the Divine Insight talent might affect that, since if it proccs as you use Halo, you then have to cast DP before being able to cast MB.
When the boss is below 20%, you'll probably just hit DP everytime you reach 3 Orbs, since you'll be generating them so fast.