Largely down to regulation changes between seasons - e.g. if they ban traction control then you need to rethink your aerodynamic package and suspension etc. Then there's the occasional complete paradigm shifts - V10 to V8, KERS, turbocharging. In seasons with little change to the regs there's also the fact that the top teams will have had all their advantages figured out over the previous season and need to change a seemingly perfect design to stay ahead of the crowd.
Largely down to regulation changes between seasons - e.g. if they ban traction control then you need to rethink your aerodynamic package and suspension etc. Then there's the occasional complete paradigm shifts - V10 to V8, KERS, turbocharging. In seasons with little change to the regs there's also the fact that the top teams will have had all their advantages figured out over the previous season and need to change a seemingly perfect design to stay ahead of the crowd.