Because if someone was planting IEDs, they should be prominently visible, and have lights drawing attention to it...
What IED handbook would these people be reading?
Oh wait, maybe it's the handbook that says "Make them look like they're just for entertainment, so everyone will think they're just harmless marketing gimmicks.". But if so, the handbook should specify they should make it Mickey Mouse, not some obscure TV show...
From an attacker perspective, drawing victims closer to the device before detonation would increase the lethality.
But if it’s placed on a bridge it doesn’t really matter, your target is the people above on the bridge right? If anything in that circumstance it seems like it would make it more likely to be discovered and stop your plan.