I see no cables actually get routed through the Suez canal, even though many cables come up the Red Sea. The same is true for the Panama canal. Is it because they are too shallow? Maybe too concentrated?
I think it's due to accessibility and the way the canals work. The Panama canal itself is actually a series of waterways connected via gates. Allowing ships to be raised and lowered as they need to change elevation. I doubt that'd be very suitable conditions for laying cable. Far easier to make connections to land based routes on either side of the isthmus.
As the Stephenson article linked elsewhere in this thread says, cablelayers hate and fear shipping lanes. Anchors kill underwater cables dead. Additionally, canals have to be dredged fairly often, which are similarly incompatible with seafloor communications cables.
