We apparently don't know why, but we're pretty sure about who started the custom.
- "Re: Hello
The Phrase Finder (October 3, 2002)
- "...Surprisingly enough, 'hello' didn't become a truly common greeting until the mid 1860s...."
- "Mark Twain Satirizes 'A Telephonic Conversation' "
- "...Alexander Graham Bell first exhibited his telephone at the Centennial Exposition in Philadelphia in 1876, but many people were initially dubious about the utility of Bell's invention. Nevertheless, by the mid-1890s, about 300,000 phones were in use and by World War I, the number reached 10.5 million. Learning to use this new device, Americans wondered what to say to start a telephone conversation. Bell's choice for an initial greeting was 'Ahoy.' Others argued for more formal greetings like 'What is wanted?' or 'Are you there?' In 1877, Thomas Alva Edison, the famous inventor who developed the first practical telephone transmitter, solved the problem by introducing 'Hello!' as the standard English telephone greeting...."
- A bit after that, History Matters does get around to Mark Twain's essay.