I think that when they sat down and tried to figure out a neat way to do this they quickly realised that legacy support=more code=more bugs=more exploits and less reliability.

IPv6 was a chance to re-imagine the internet all over again. (And lets face it, the internet is still in it infancy, we don't even have a full set of working browsers yes. ), and they have done everything they can to make sure these problems don't come up again.