It's also far more prone to damage from flooding and tremendously more difficult to troubleshoot and repair. It works well in areas that are dry
meanwhile in the uk we run 400KV cables under the towpath and use the canal water to keep the cables cool
How many junctions, switches, fuses, and transformer vaults are down there under the canal water? How much does it cost to maintain them? Do you think the UK possesses some magical technology that North America lacks? Keep in mind the entire country is smaller than some of our individual states, there has got to be a lot less electrical infrastructure, vastly fewer km of cable.
This is one source I came across: "According to the May 2011 paper “Underground Electric Transmission Lines” published by the Public Service Commission of Wisconsin, “The estimated cost for constructing underground transmission lines ranges from 4 to 14 times more expensive than overhead lines of the same voltage and same distance."
Underground lines have their place, they have advantages, but they also have disadvantages. Because of that we typically use underground in cities and modern suburbs but the long runs between towns are usually overhead.