That would be the more general/traditional way of saying it, but in modern investment circles the focus seems to have turned towards the actual people being "bu

That would be the more general/traditional way of saying it, but in modern investment circles the focus seems to have turned towards the actual people being "bulls/bears" and not just the attitudes of the market. A person is a bull or a bear, as opposed to a person being either bullish or bearish. So in this construction, a "bull case" is a "case that a bull (the person) can make".