IBM was the entire IT industry of 1950s, 1960s, and most of 1970s. It is hard to explain IBM's dominance in today's terms. IBM used to be so dominant that the entire financial industry could not operate without their IBM mainframes and today major banks still run on IBM.

The downside (to IBM) was that Wall Street decided "never again" and fought hard to prevent another company of IBM's scale from reappearing.