Well I think there are several factors that might explain that.
First of all they made about 2.5x more 458 than they have 430s.
Second, the 430 offers amazing value for money at the moment, and people might not want to part from them as their cars are worth more to them than what they can sell them for.
As way of example, from personal experience, I was looking at at F12 from HR Owen, and asked them to give me a PX quote for my 2008 red/tan 430 w FFSH and 20k miles and they came back with £54k.... that F12 was 6 y/o and £100k more than my 430... not worth it in my books. Same goes for a 458, I’d need to add £60-70k to get into a car that wouldn’t offer me 2x the fun and probably a less of a raw / auditory experience.