Two things:

1. There have been shamans that rot first, then leave the fight. Clearly not all shamans are like this, but it tends to color peoples' perceptions. Similarly there have been assassins who tried the assassinate then immediately fled. In that case, though, there's arguably significant risk to the assassin, since he has to be in bash range to even attempt. With the shaman, he can protect himself against most lag, and withstand what he can't protect against. So to the person being rotted, the shaman is killing him with no real benefit to the shaman (since he's not getting the corpse) and with no real risk to himself. People dislike that. Assassinate also requires some setup effort; not so with rot.

2. Perceived inevitability. With insects, dangerous as they are, you feel like you have a *chance* to avoid dying by running on foot. In contrast, many people feel like rot is an absolute death sentence. Clearly this isn't true in general, but in some cases (characters w/ very low con) it approaches being accurate.

3. Time drain. With fiend, you either die or you don't. Either way, you find out pretty quick. Ditto with assassinate. With rot, even if you end up surviving it, you're out of commission for approx. 24 hours game time. That tends to annoy people.