Which new form of communication put the pony express out of business after just 18 months of operation?

The telegraph

Explanation:

It took about 10 days and many relay riders to cover the distance between Missouri and California, so even if the pony express was a relatively fast was of sending letters, it still took a long time. On the other hand, the telegraph was almost instantaneous. You needed to set up the communication lines and someone who understood the Morse code, but the difference in time was huge.