e5500(or e6500, for that matter) based SoCs only support USB 2.0. Since it's not a consumer oriented product, it's not a big drawback for it's intentional market and primary use.
not to mention that, at least for the moment, neither AOS4 nor MOS support USB 3.0. and once that changes, we can always add a PCIe card for that.

-- eliyahu