It has nothing to do with science or even power generation. This is about regulatory compliance. They put them over canals because a canal is not private land

It has nothing to do with science or even power generation. This is about regulatory compliance. They put them over canals because a canal is not private land. Either they are owned by government or the government has easements for their maintenance and improvement. Canals are also readily accessible for light construction, most all of them having a dirt road alongside. The same principals apply to roads. Governments face all sorts of issues acquiring and building upon new land. But placing something beside, on top, or even within the road surface can normally be done without new permits let along environmental impact statements.