Short for because it's time network, BITNET was a U.S. network of IBM mainframe systems founded in 1981. It was first established between the University of New York and the Yale University. In 1986, BITNET II was created to help bandwidth needs, which merged with CSNET (Computer Science Network) to form CREN (Corporation for Research and Educational Networking) in 1988.
As the Internet became popular in the 1990s, BITNET's usage declined and became defunct and is no longer in use today.
When it was created, BITNET was short for because it's there network.