What caused the end of pony express?

The telegraph line was built, thus making the Pony Express impractical, as it took more time, energy, and money to run.

The final nail in the coffin of the Pony Express was completion of the transcontinental telegraph at Salt Lake City on October 24, 1861. Two days later, the Pony Express ceased operations. The great experiment was over. A major American enterprise had failed, but it was hardly the end of the world.