The bottom of the canal isnt tiled. Its made of stuff you sink into. Also, falling in is an involuntary action leading to shock, gasping for air, scrambling about etc. Hard to get footing I should I think. Its also dark, alcohol possibly involved and other factors. It is far from as easy as just standing up, otherwise nobody would die even if it was a shallow part. People also dont realise the effect cold water has on the body especially at night. People cant get out, unfortunately its as simple as that