Why? I replace mine as and when they need it, don't rotate them at all. At the minute I have different tyres front and rear as well.
Because the rear wheels are the ones over which you have the least control (in most cases). Most people's justification for placing the newer tyres on the front is "because the front wheels do the driving, steering and the braking". But this is precisely the reason the newer tyres should on the rear - you have more control over what the front wheels are doing. You can modulate the brakes, steer, and (for FWD) adjust the the throttle in order to control traction.
By comparison, you have a lot less control over the rear wheels and about the only factor you can influence in regards to grip is the quality of the tyres. Therefore, in a situation where you are only replacing tyres in pairs, it makes sense to always have the newer tyres on the rear.
Of course in the ideal world, you'd replace all 4 tyres at the same time, but this isn't practical, and as most tyres wear unevenly in pairs, people will replace one pair at a time. So it's just about keeping as much control over the factors which influence stability as possible.
EDIT: a supplementary reason is tyre age. Most people replace tyres based on their mileage, which is sensible. But tyres can also lose effectiveness as they age. If you drive a FWD car and are replacing the fronts regularly, but not the rears, then those rear tyres might still be of legal tread depth, but older than the fronts. I accept this is a bit of an outlier reason though, I doubt many cars burn through their front tyres so quickly that you'd replace them multiple times before the rears needed doing.
no go here
staggered fitment on both cars so tyre spec is different.. I rarely keep wheels long enough to burn through tyres and end up getting new set with fresh rubber..
..except of course if you have a staggered setup
