Perhaps Ricoh's decision to go with USB2 wasn't based on one reason but several. USB 3 is supposed to be backwards compatible but that isn't always the case, especially in older computers. Chances are nearly 100% of end users have access to USB 2 but the percentage for USB 3.0 is probably 1/2 that or less. USB 3.0 has also been superseded by USB 3.1 gen 1 and 2, and USB C.
While USB 3 would have been nice, if USB 2 is a deal breaker for you, then you are in a very very
small group, considering all the other features Ricoh put into the K1 especially at the price they have established. To paraphrase a well known saying, You can please some of the people some of the time, and some of the people all of the time, but you can't please all of the people, all of the time.