As seen in the tire types on previous pages, rubber comes in many forms. Usually the type of rubber is paired with tire design to create a purpose, and again, it's on a spectrum running from durability and gas mileage to performance.
Basically, soft rubber sticks and hard rubber lasts.
This is partly due to off-gassing. As a tire heats up, the rubber literally turns from a solid to a gas. The rubber in a softer track tire may only be designed to off-gas three or four times before the once-pliable rubber turns brittle and your tire is toast. That's an expensive habit for the road rider. On the other hand, the rubber of a cruiser tire is designed to hold its gas. It can heat and cool many times before it's gassed out.
If you plan to ride in the rain, considering ponying up for a silica-enriched compound, which is grippy when wet.