Let me explain a bit more in detail, as my original post wasn't that well developed. Let's start with our Hawx boot, which is a 100mm last. The internal volume of this shell is very conducive to having a liner construction that offers a great blend of performance materials and insulation (foams, fleeces, etc). Simply put, there is enough space in the shell to offer both.
Let's move now to the Redster Pro, which is a 98mm last. This drop of 2mm represents a big loss of internal volume, which now means you now have less space in the shell to build a liner. If you put the same liner construction of the Hawx (but lasted for Redster Pro) in the Redster Pro shell, the boot would feel like a 95mm boot and simply too tight for most people to wear. So, to make it feel like a 98mm boot, we have to take some higher volume (warmer) materials out of the liner. The Redster Pro liner uses super high end but thinner materials. While high end, they are still thinner and therefore allow the foot to sit closer to the shell. This delivers more performance but also sacrifices a bit of warmth when compared to a 100mm boot. There simply isn't the space available inside the shell to offer the same degree of warmth that a Hawx boot provides.
Let's now move to Redster World Cup, which is our 95mm last for pure race applications. This liner is absolutely bare bones in terms of insulation. The materials in this liner are selected to provide optimum snow feel, terrain feedback, and foot hold. Normal "thick" insulation is completely removed because it provides too much cushioning and keeps the foot further away from the shell, thus reducing snow feel and terrain feedback that the racer desperately needs.
As Tom said, you could make a Hawx boot with an ultra thick liner so it felt like a 98mm boot, but then it would simply pack out after a while and become sloppy.