USB 1 was slow because it was designed to be a cost-effective way to connect keyboards, mice, etc. result: it did not require high-quality cabling, had the PC poll all devices for data, etc.
USB 2 stretched the USB 1 interface to its limits (for the technology of the day, and for a desired cost level). Devices could indicate that they could handle higher-frequency signalling and if so, they would be sent such signals, Backward compatibility meant that low speed devices kept using the slower timing, even if they shared the bus with higher speed devices. So, low-speed devices still use the same fraction of the total bandwidth. For example, a mouse will want to be polled a hundred times per second. It also will want be cheap, so it will use the slowest USB speed. Result: if it took 1% of USB 1 bandwidth, it still takes 1% of USB 2 bandwidth, while USB 2 is a lot faster. And because devices have to be polled, that is regardless of whether you use your mouse (yes, your mouse can take 4Mbps of your USB 2 bandwidth)
If USB 3 went along this line, things would have gotten ludicrous. Because of that, USB 3 uses different cabling. That allowed the, to also fix some issues that USB had. For instance, devices can now signal that they have data to deliver, whereas they had to be polled before.
Summary: USB 3 is a totally new protocol that uses different (higher quality) cabling and only shares some terminology with USB 2.
USB 2 stretched the USB 1 interface to its limits (for the technology of the day, and for a desired cost level). Devices could indicate that they could handle higher-frequency signalling and if so, they would be sent such signals, Backward compatibility meant that low speed devices kept using the slower timing, even if they shared the bus with higher speed devices. So, low-speed devices still use the same fraction of the total bandwidth. For example, a mouse will want to be polled a hundred times per second. It also will want be cheap, so it will use the slowest USB speed. Result: if it took 1% of USB 1 bandwidth, it still takes 1% of USB 2 bandwidth, while USB 2 is a lot faster. And because devices have to be polled, that is regardless of whether you use your mouse (yes, your mouse can take 4Mbps of your USB 2 bandwidth)
If USB 3 went along this line, things would have gotten ludicrous. Because of that, USB 3 uses different cabling. That allowed the, to also fix some issues that USB had. For instance, devices can now signal that they have data to deliver, whereas they had to be polled before.
Summary: USB 3 is a totally new protocol that uses different (higher quality) cabling and only shares some terminology with USB 2.