I recently did a lot of reading on the Panama Canal, and some of that info will shed some light on this.

During westward expansion, many people, like the Donner Party, travelled cross-country as part of a wagon train. This usually consisted of one or more wagons carrying provisions and other supplies, some livestock, people on horseback, and people walking. Most of these people lacked the training in cartography or the inclination to make maps. Anecdotally, I was just talking with a friend the other night who used to work as a land-man (basically they research mineral rights so companies can mine or drill for oil or other minerals). He told a story of one old land deed that marked its borders by stating the distance from one point to another in terms of how far he rode after smoking two cigarettes. So that’s a major reason it’s uncharted.

Then, in 1848-1849, someone found gold in California, causing the so-called Gold-Rush. Some travelled west in the aforementioned wagon trains. The transcontinental railroad didn’t open until 1869, so that wasn’t an option yet. Most people went south, instead of west. Of those people, many sailed all the way to Cape Horn at the tip of South America and then up north along the Pacific coast until they reached San Francisco. Others debarked in Panama and travelled overland (the narrowest point of the Isthmus of Panama is about 50 miles. This is where the Panama Canal would eventually be built). Many of the people traveling this way ran afoul of the local population and some trouble ensued, prompting the creation of the Panama Railroad, which opened in 1855. The short overland route ended in Panama City, where travelers would catch a steamer north to San Francisco. This route was the fastest at the time, but the overland journey between 1848-1855 was difficult, through thick jungle, over some small mountains, across a large river notorious for its flooding (the Chagres River). Disease was also a massive problem, and would remain so until about 1908, when Dr Gorgas of the American Canal team effectively eradicated mosquito borne disease from the region.

While California was already somewhat settled before the gold rush, the massive influx of people and money (from gold) helped the population boom immensely. Despite the difficult overland route across North America, shipping was still available, it just took a longtime from coast to coast. This was the main reason so many people wanted the Canal. Before the US finally pulled it off in 1914, the French had already lost billions of investors francs trying to build the Canal under the leadership of Ferdinand de Lesseps, Le Grand Francais. He had built the Suez Canal in Egypt, so he assumed, wrongly, that building a seas level Canal across the isthmus would be no more difficult. By the time the Canal opened (literally the day after World War 1 started) the West was mapped, the transcontinental railroad had been open for 50 years, and nobody really noticed the Canal opening because the world was at war.

Most of my info is from The Path Between the Seas by David McCullough. It’s a great book on the complete history of the Canal. I’m also trying to hunt down another source I have around here that deals specifically with the problems with the 49ers in Panama. I’ll update when I find it. Hope that all helps.