Because many things can be included into what is commonly called “cardio”… you have low intensity/low duration work (LSD … long and slow distance), intensive intervals, extensive intervals, intensive segmented (400m for example), etc. You could even add GPP work like sled dragging and sledgehammer swing in the mix.
Basically all are forms of exercise used to “get in shape” and to shed some fat (the later being the most popular reason). However they do rely on different energy pathways. Cardio, refers to cardiovascular work. And I find that term too vague: any work/activity involving an increase in heart or respiratory rate could be callec cardio …
However by using the term energy system work it allows you to get more specific. E.g. Anaerobic glycolysis energy system work; oxidative energy system work, etc.
Just like we can use terms like “strenght training”, “power training”, “hypertrophy training”, “strength-endurance training” to describe weight lifting activities.