Canals are for the most part fairly photogenic. Even those directly under spaghetti junction can have some interesting things done with them. However wikipedia isn’t just interested in photos of canals looking pretty but photographs that document the canal. Since it is difficult to show a feature about 20 miles long and only around 30 foot wide this presents a bit of a problem. A typical bit of canal tends to look much the same as any other canal other than the difference between urban and rural . While those very familiar with the canals in question may be able to have a stab at telling them apart that is of limited use to the reader. Some canals do have iconic parts. The Llangollen Canal has the Pontcysyllte Aqueduct for example. Not all canals do though and iconic parts are hardly representative of the rest of the canal.
I have no solution to this problem mind although I suppose it does illustrate the challenge of taking encyclopedic pictures rather than purely ascetically pleasing ones.