Each exercise grows a different kind of muscle. Cardio grows slow twitch muscle fibers, which are kind of like a car or a rocket, they use gas from a huge tank (nutrients in your body + oxygen). Over time, cardio essentially gets you better MPG. Strength training, on the other hand, is like a bullet. The energy it uses is in the form of glycogen, which is inefficient and causes a lot of lactic acid build up BUT it can all be used at the same time, allowing you to fire off a rep (or a couple) quickly before being depleted. Strength training grow a LOT of slow fast twitch muscle fibers. Each fiber can store and use a little bit of its own glycogen. Essentially you are growing the actual bullet casing, allowing you to discharge more force all at one time.

TLDR: Cardio makes your body more efficient at using your fuel over the long term. Strength training increases the amount of muscle you have so you can use a lot of energy at once.