Dont forget that humanity has been tinkered with by the puppeteers. Incredible risk is inherent in space exploration and colonization, and most of humanity is risk averse. Alos humanity is limited in expansion power by the tech that the Outsiders and General Products sell them. In the later stories I think that the only reasons that humanity expanded at all was because of the wars with the Kzin, and those wars were won because the Puppeteers messed with the Kzin too. Those pressures to maintain the status quo by the Puppeteers are pretty considerable too. In short, its not really technology that is holding humanity back. Its a mind set and the fact that the psychological needs have been redefined, and those that humans have are satisfied by aliens who don't want us to expand.