To over-simplify: TTs tend to be flatter, so less of the rider's power is devoted to overcoming gravity and more of it is devoted to going very fast in a (relatively) straight line. Skipping past square-cube laws and aerodynamics, power in a TT is largely spent on overcoming wind resistance, which makes it more about absolute power rather than relative power.
Emma Pooley, who knows a thing or two about both climbing and TTs, explains it better