An incendiary device in the tunnel would make VERY good headlines. A severe fire would close both bores for some weeks and the affected one for months. Think about the fires in the Alpine road tunnels.
There was such a fire in the tunnel not long after it opened. It occurred on a lorry shuttle. It did indeed close one bore for some time, but it didn't cause massive service disruption and it killed precisely no (0) passengers, and didn't cause any injuries bar slight smoke inhalation, as I recall.
It *is* a relatively high-profile target, but there are far "better" choices. That, most likely, will be why no terrorist has yet attacked it.