The scope of HCI has changed because how people use computers has also changed dramatically since the 1980s, when the CHI conference started.

The disciplines that fall under the HCI umbrella has also changed dramatically. In the 1980s, it was cognitive psychologists and computer scientists. Social psychology came in, then design, then ethnography, developing countries, social justice, public policy, and many many more. Having been in the field for almost 30 years, I think that this breadth is part of HCI's strength, though also it's weakness.

Aside: I saw Tom Mitchell (of Machine Learning fame) give a talk about how the field evolved over the past 40 years. What I found fascinating is that, for the most part, there was a single question that drove the field: how to make computers learn from experience (or data)? However, there isn't a single such question for HCI. Again, it's part of its strength and its weakness.