CJ3 and CJ4 are over 5.7T and are single pilot aircraft. I think its just to do with the single/multipilot certification.
I'd be interested to see the statistics on the safety record of single pilot type jets operated single pilot and see if it is demonstrably less safe that operating those aircraft two crew. I can't think of any recent serious single pilot instances in jets but there have been a few in single pilot types operated multi crew. I've never understood the logic of allowing single pilot operations in a kingair or similar but not in a CJ or a Premier1 - both of which are much simpler aircraft with better avionics (generally) and much better OEI performance.
Its difficult to compare like with like when analysing accident statistics so I don't know how easy it is to draw conclusions regarding the relative safety of single pilot flying.