Because at lift off oversteer the back will be extra slidy and also under extremely heavy braking, as the weight transfers to the front, the rear will have less grip in relation to the front and if you've got grippier tyres at the front, it will have even less relative grip.
Someone on here recently had new tyres only on the front and crashed it soon after, not saying that's the only reason but from memory he was on a spirited drive no different to normal.
I've always had my tyres rotated so if I've needed new fronts, I'll put the rears on the front and have new tyres on the rear. If you've got a lot of grip on the rear and are getting tyres of similar grip levels anyway then very little issue having new straight on the front. If you're going to get much grippier tyres, I would put them on the rears IMO. I've always thought this was the general thought with tyres and FWD cars.
