You start to thirst for something faster. You have begun to regress into your modem state again. You feel the need for speed. This is a question that I'm sure many ponder. Well there are many reasons, but the most common would be local traffic and Internet traffic period.
Local traffic is the biggest speed cap that plagues cable modems.
Depending on where you live, you could have a busy grid, or a not so busy one. But others may not get much speed at all because there are lots of people signed up in their area using up a percentage of that T3's resources, causing it to slow down. This is the biggest fallback of cable Internet technology. The problem lies with the cable companies to upgrade their equipment to meet the needs of the consumer. There isn't much you can do besides maybe write angry letters.