ELI5 Panama Canal - why did the original French attempt fail?

What makes the Panama Canal’s lock system so essential compared to the Suez Canal’s lack of locks? How do locks help manage the canal’s significant elevation change across mountainous terrain? Why was constructing a wide trench connecting the Pacific and Atlantic oceans deemed too complicated and risky? Why did the U.S. ultimately choose Panama for the canal after initially considering Nicaragua as a better option? What historical challenges did the French face while attempting to build the Panama Canal before the U.S. took over?

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