What does an MCU need to be capable of USB 3.0 communication?
USB 3.0 operates at 5Gbit/sec. Not only does that require high-frequency clock generation, for many "microcontroller" level products it exceeds the RAM bandwidth! There's simply no point in connecting to a firehose you cannot make use of.

This TI processor with USB3 has DDR3 support, which is fast enough, and multiple >1GHz cores to actually make use of that data.