Because the grip is better, especially with sweaty hands (like myself), and the magazine button is now possible for many people to press without having to alter their grip. The Glock 30, if I'm not mistaken, already has a dual recoil spring assembly, right? So there's no need to worry about that changing. I'd personally love a Gen 4 Glock 30 as well.
I love Glocks and I've shot them for quite a while also, but since the Gen 4 texture came around I just can't go back to Gen 3. My mitts sweat a lot when I'm at the range, and the Gen 3 pistols force me to adjust my grip constantly.