If you look at USB 3.0 connector, or better measure interconnect parameters, you will see not much of a difference. In both cases the differential impedance, although designed for 90 Ohms, will have some discontinuity, which will depend even on how do you bend the mating plug. The difference however is that USB 3.0 frequency is 10x higher than USB2, and that's makes the signal quality much more susceptible to all imperfections.
So in short, a USB 3.0 port fails to start due to questionable cable quality.
One of most questionable part of USB cable is situated in the cable overmold. USB cables are not designed to be assembled in well-controlled automated way, they require manual labor to solder split leads of bulk cable onto connector's terminals. The leads might be bent and go wide, solder blobs night differ in size, etc., thus introducing an inhomogeneity into transmission line. This is in addition to connector deficienies. As result, bit patterns of USB 3.0 signals scatter over these "bumps" and "dips", interfere, reflect back, and make the signal eye ugly and barely-decodable.
The traces between USB connector and host chip also are not perfect, and soldered connector almost always is a "bump" in the channel. More, longer cables tend to attenuate high frequencies more, so the signal loses sharpness of edges and amplitude drops. Altogether this forms "lossy communication channel", in full similarity to RF communication. In some cases the impedance imperfections at connection points might form an anti-resonant condition, resulting in substantial loss of signal amplitude. A cable inch longer or inch shorter might however work nearly fine.
In attempt to correct the "channel" properties, USB 3.0 signals have "pre-emphasis" on transmission end, and tunable equalizing filter on receiver end.
To make the channel work, USB 3 employs "link training", by sending 65536 special training packets. The receiver selects the best filter parameters based on minimum error level. If the channel has too many reflections or too attenuated, the training would fail, and the USB3 port will be turned into disabled.
The other scenario would be if the link training gets passed, and link switched to active "U0" mode, the USB protocol might have too many errors and fails to complete transactions. In this case the host will try to "reset" and re-train the link, but results will likely be the same. After several attempts the host driver will disable the USB3 part.
Once USB3 link fails, USB device may (or may not) engage the USB 2.0 connect protocol.
In summary, it is nearly impossible to "diagnose" cable specific issues without taking signal quality measurements using special test fixtures and fairly-high bandwidth scopes (8-12 GHz) and TDR instruments, with special software packages. The best way is to work with all three components of the link (host-cable-device) that are USB-IF certified.