With an uninformed and wild guess ... the Chunnel is not such a tempting target as many folks in the countries that the like of OBL wish to impress - do not know that the Chunnel exists.
You want to be able to show newspaper and TV pictures of an internationally known symbol - blown up or brought down. In the UK, there are a number of buildings and locations that are known all around the world from movies and educational text books. In Paris, they would rather do mild damage to the Eiffel Tower than utterly destroy a mainline railway station. Destroying the station would upset the country more from an economical point of view but the tower is their iconic emblem.
Of course, as was shown in Madrid, the folks will take what they can get - so we must assume that everything is a target. That, of itself, means that they have achieved a major objective. The IRA famously put the terrorist's point of view when they failed to kill Thatcher.