I certainly don't consider myself an expert but IMO the possible factors to take into consideration would be:
1) in the latter half of 40 the French Atlantic bases were operational
2) increasing Allied air sueriority as the war progressed
3) due to the shallow depth of the canal transits would have to be on the surface
4) obvious choke point for mines
just my opinion because as posted earlier there aint much out there on the internet about this topic
